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ZEO Documentation
=================
This directory contains ZEO documentation.
howto.txt
The first place to look.
It provides a high-level overview of ZEO features and details on
how to install and configure clients and servers. Includes a
configuration reference.
cache.txt
Explains how the client cache works.
trace.txt
Describe cache trace tool used to determine ideal cache size.
ZopeREADME.txt
A somewhat dated description of how to integrate ZEO with Zope.
It provides a few hints that are not yet in howto.txt.
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Zope Enterprise Objects (ZEO)
Installation
ZEO 2.0 requires Zope 2.4 or higher and Python 2.1 or higher.
If you use Python 2.1, we recommend the latest minor release
(2.1.3 as of this writing) because it includes a few bug fixes
that affect ZEO.
Put the package (the ZEO directory, without any wrapping directory
included in a distribution) in your Zope lib/python.
The setup.py script in the top-level ZEO directory can also be
used. Run "python setup.py install --home=ZOPE" where ZOPE is the
top-level Zope directory.
You can test ZEO before installing it with the test script::
python test.py -v
Run the script with the -h option for a full list of options. The
ZEO 2.0b2 release contains 122 unit tests on Unix.
Starting (and configuring) the ZEO Server
To start the storage server, go to your Zope install directory and
run::
python lib/python/ZEO/start.py -p port_number
This run the storage sever under zdaemon. zdaemon automatically
restarts programs that exit unexpectedly.
The server and the client don't have to be on the same machine.
If they are on the same machine, then you can use a Unix domain
socket::
python lib/python/ZEO/start.py -U filename
The start script provides a number of options not documented here.
See doc/start.txt for more information.
Running Zope as a ZEO client
To get Zope to use the server, create a custom_zodb module,
custom_zodb.py, in your Zope install directory, so that Zope uses a
ClientStorage::
from ZEO.ClientStorage import ClientStorage
Storage = ClientStorage(('', port_number))
You can specify a host name (rather than '') if you want. The port
number is, of course, the port number used to start the storage
server.
You can also give the name of a Unix domain socket file::
from ZEO.ClientStorage import ClientStorage
Storage = ClientStorage(filename)
There are a number of configuration options available for the
ClientStorage. See doc/ClientStorage.txt for details.
If you want a persistent client cache which retains cache contents
across ClientStorage restarts, you need to define the environment
variable, ZEO_CLIENT, or set the client keyword argument to the
constructor to a unique name for the client. This is needed so
that unique cache name files can be computed. Otherwise, the
client cache is stored in temporary files which are removed when
the ClientStorage shuts down. For example, to start two Zope
processes with unique caches, use something like::
python z2.py -P8700 ZEO_CLIENT=8700
python z2.py -P8800 ZEO_CLIENT=8800
Zope product installation
Normally, Zope updates the Zope database during startup to reflect
product changes or new products found. It makes no sense for
multiple ZEO clients to do the same installation. Further, if
different clients have different software installed, the correct
state of the database is ambiguous.
Zope will not modify the Zope database during product installation
if the environment variable ZEO_CLIENT is set.
Normally, Zope ZEO clients should be run with ZEO_CLIENT set so
that product initialization is not performed.
If you do install new Zope products, then you need to take a
special step to cause the new products to be properly registered
in the database. The easiest way to do this is to start Zope
once with the environment variable FORCE_PRODUCT_LOAD set.
The interaction between ZEO and Zope product installation is
unfortunate.
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ZEO Client Cache
The Client cache provides a disk based cache for each ZEO client.
The client cache allows reads to be done from local disk rather than
by remote access to the storage server.
The cache may be persistent or transient. If the cache is
persistent, then the cache files are retained for use after process
restarts. A non-persistent cache uses temporary files that are
removed when the client storage is closed.
The client cache is managed as two files. The cache manager
endeavors to maintain the two files at sizes less than or equal to
one half the cache size. One of the cache files is designated the
"current" cache file. The other cache file is designated the "old"
cache file, if it exists. All writes are done to the current cache
files. When transactions are committed on the client, transactions
are not split between cache files. Large transactions may cause
cache files to be larger than one half the target cache size.
The life of the cache is as follows:
- When the cache is created, the first of the two cache files is
created and designated the "current" cache file.
- Cache records are written to the cache file, either as
transactions commit locally, or as data are loaded from the
server.
- When the cache file size exceeds one half the cache size, the
second cache file is created and designated the "current" cache
file. The first cache file becomes the "old" cache file.
- Cache records are written to the new current cache file, either as
transactions commit locally, or as data are loaded from the
server.
- When a cache hit is found in the old cache file, it is copied to
the current cache file.
- When the current cache file size exceeds one half the cache size, the
first cache file is recreated and designated the "current" cache
file. The second cache file becomes the "old" cache file.
and so on.
Persistent cache files are created in the directory named in the
'var' argument to the ClientStorage (see ClientStorage.txt) or in
the 'var' subdirectory of the directory given by the INSTANCE_HOME
builtin (created by Zope), or in the current working directory.
Persistent cache files have names of the form::
cstorage-client-n.zec
where:
storage -- the storage name
client -- the client name, as given by the 'ZEO_CLIENT' environment
variable or the 'client' argument provided when creating a client
storage.
n -- '0' for the first cache file and '1' for the second.
For example, the second cache file for storage 'spam' and client 8881
would be named 'cspam-8881-1.zec'.
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==========================
Running a ZEO Server HOWTO
==========================
Introduction
------------
ZEO (Zope Enterprise Objects) is a client-server system for sharing a
single storage among many clients. Normally, a ZODB storage can only
be used by a single process. When you use ZEO, the storage is opened
in the ZEO server process. Client programs connect to this process
using a ZEO ClientStorage. ZEO provides a consistent view of the
database to all clients. The ZEO client and server communicate using
a custom RPC protocol layered on top of TCP.
There are several configuration options that affect the behavior of a
ZEO server. This section describes how a few of these features
working. Subsequent sections describe how to configure every option.
Client cache
~~~~~~~~~~~~
Each ZEO client keeps an on-disk cache of recently used objects to
avoid fetching those objects from the server each time they are
requested. It is usually faster to read the objects from disk than it
is to fetch them over the network. The cache can also provide
read-only copies of objects during server outages.
The cache may be persistent or transient. If the cache is persistent,
then the cache files are retained for use after process restarts. A
non-persistent cache uses temporary files that are removed when the
client storage is closed.
The client cache size is configured when the ClientStorage is created.
The default size is 20MB, but the right size depends entirely on the
particular database. Setting the cache size too small can hurt
performance, but in most cases making it too big just wastes disk
space. The document "Client cache tracing" describes how to collect a
cache trace that can be used to determine a good cache size.
ZEO uses invalidations for cache consistency. Every time an object is
modified, the server sends a message to each client informing it of
the change. The client will discard the object from its cache when it
receives an invalidation. These invalidations are often batched.
Each time a client connects to a server, it must verify that its cache
contents are still valid. (It did not receive any invalidation
messages while it was disconnected.) There are several mechanisms
used to perform cache verification. In the worst case, the client
sends the server a list of all objects in its cache along with their
timestamps; the server sends back an invalidation message for each
stale object. The cost of verification is one drawback to making the
cache too large.
Note that every time a client crashes or disconnects, it must verify
its cache. Every time a server crashes, all of its clients must
verify their caches.
The cache verification process is optimized in two ways to eliminate
costs when restarting clients and servers. Each client keeps the
timestamp of the last invalidation message it has seen. When it
connects to the server, it checks to see if any invalidation messages
were sent after that timestamp. If not, then the cache is up-to-date
and no further verification occurs. The other optimization is the
invalidation queue, described below.
Invalidation queue
~~~~~~~~~~~~~~~~~~
The ZEO server keeps a queue of recent invalidation messages in
memory. When a client connects to the server, it sends the timestamp
of the most recent invalidation message it has received. If that
message is still in the invalidation queue, then the server sends the
client all the missing invalidations. This is often cheaper than
perform full cache verification.
The default size of the invalidation queue is 100. If the
invalidation queue is larger, it will be more likely that a client
that reconnects will be able to verify its cache using the queue. On
the other hand, a large queue uses more memory on the server to store
the message. Invalidation messages tend to be small, perhaps a few
hundred bytes each on average; it depends on the number of objects
modified by a transaction.
Transaction timeouts
~~~~~~~~~~~~~~~~~~~~
A ZEO server can be configured to timeout a transaction if it takes
too long to complete. Only a single transaction can commit at a time;
so if one transaction takes too long, all other clients will be
delayed waiting for it. In the extreme, a client can hang during the
commit process. If the client hangs, the server will be unable to
commit other transactions until it restarts. A well-behaved client
will not hang, but the server can be configured with a transaction
timeout to guard against bugs that cause a client to hang.
If any transaction exceeds the timeout threshold, the client's
connection to the server will be closed and the transaction aborted.
Once the transaction is aborted, the server can start processing other
client's requests. Most transactions should take very little time to
commit. The timer begins for a transaction after all the data has
been sent to the server. At this point, the cost of commit should be
dominated by the cost of writing data to disk; it should be unusual
for a commit to take longer than 1 second. A transaction timeout of
30 seconds should tolerate heavy load and slow communications between
client and server, while guarding against hung servers.
When a transaction times out, the client can be left in an awkward
position. If the timeout occurs during the second phase of the two
phase commit, the client will log a panic message. This should only
cause problems if the client transaction involved multiple storages.
If it did, it is possible that some storages committed the client
changes and others did not.
Monitor server
~~~~~~~~~~~~~~
The ZEO server updates several counters while it is running. It can
be configured to run a separate monitor server that reports the
counter values and other statistics. If a client connects to the
socket, the server send a text report and close the socket
immediately. It does not read any data from the client.
An example of a monitor server report is included below::
ZEO monitor server version 2.1a1
Fri Apr 4 16:57:42 2003
Storage: 1
Server started: Fri Apr 4 16:57:37 2003
Clients: 0
Clients verifying: 0
Active transactions: 0
Commits: 0
Aborts: 0
Loads: 0
Stores: 0
Conflicts: 0
Conflicts resolved: 0
Connection management
~~~~~~~~~~~~~~~~~~~~~
A ZEO client manages its connection to the ZEO server. If it loses
the connection, it starts a thread that attempts to reconnect. While
it is disconnected, it can satisfy some reads by using its cache.
The client can be configured to wait a connection when it is created
or to return immediately and provide data from its persistent cache.
It usually simplifies programming to have the client wait for a
connection on startup.
When the client is disconnected, it polls periodically to see if the
server is available. The rate at which it polls is configurable.
The client can be configured with multiple server addresses. In this
case, it assumes that each server has identical content and will use
any server that is available. It is possible to configure the client
to accept a read-only connection to one of these servers if no
read-write connection is available. If it has a read-only connection,
it will continue to poll for a read-write connection. This feature
supports the Zope Replication Services product,
http://www.zope.com/Products/ZopeProducts/ZRS. In general, it could
be used to with a system that arranges to provide hot backups of
servers in the case of failure.
Authentication
~~~~~~~~~~~~~~
ZEO supports optional authentication of client and server using a
password scheme similar to HTTP digest authentication (RFC 2069). It
is a simple challenge-response protocol that does not send passwords
in the clear, but does not offer strong security. The RFC discusses
many of the limitations of this kind of protocol. Note that this
feature provides authentication only. It does not provide encryption
or confidentiality.
The challenge-response also produces a session key that is used to
generate message authentication codes for each ZEO message. This
should prevent session hijacking.
Guard the password database as if it contained plaintext passwords.
It stores the hash of a username and password. This does not expose
the plaintext password, but it is sensitive nonetheless. An attacker
with the hash can impersonate the real user. This is a limitation of
the simple digest scheme.
The authentication framework allows third-party developers to provide
new authentication modules.
Installing software
-------------------
ZEO is distributed as part of the ZODB3 package and with Zope,
starting with Zope 2.7. You can download it from:
- http://www.zope.org/Products/ZODB3.2, or
- http://www.zope.org/Products/Zope
To use ZEO with Zope 2.6, download ZODB3.2 and install it into your
Zope software home. ZODB3 comes with a distutils setup.py script.
You can use the --home option to setup.py install to the software in
custom location. For example, if Zope is installed in /home/zope,
then this command will install the new ZEO and ZODB:
python setup.py install --home /home/zope
The install command should create a /home/zope/lib/python/ZEO directoy.
Configuring server
------------------
The script runzeo.py runs the ZEO server. The server can be
configured using command-line arguments or a config file. This
document describes only describes the config file. Run runzeo.py
-h to see the list of command-line arguments.
The runzeo.py script imports the ZEO package. ZEO must either be
installed in Python's site-packages directory or be in a directory on
PYTHONPATH.
The configuration file specifies the underlying storage the server
uses, the address it binds, and a few other optional parameters.
An example is::
<zeo>
address zeo.example.com:8090
monitor-address zeo.example.com:8091
</zeo>
<filestorage 1>
path /var/tmp/Data.fs
</filestorage>
<eventlog>
<logfile>
path /var/tmp/zeo.log
format %(asctime)s %(message)s
</logfile>
</eventlog>
This file configures a server to use a FileStorage from
/var/tmp/Data.fs. The server listens on port 8090 of zeo.example.com.
It also starts a monitor server that lists in port 8091. The ZEO
server writes its log file to /var/tmp/zeo.log and uses a custom
format for each line. Assuming the example configuration it stored in
zeo.config, you can run a server by typing::
python /usr/local/bin/runzeo.py -C zeo.config
A configuration file consists of a <zeo> section and a storage
section, where the storage section can use any of the valid ZODB
storage types. It may also contain an eventlog configuration. See
the document "Configuring a ZODB database" for more information about
configuring storages and eventlogs.
The zeo section must list the address. All the other keys are
optional.
address
The address at which the server should listen. This can be in
the form 'host:port' to signify a TCP/IP connection or a
pathname string to signify a Unix domain socket connection (at
least one '/' is required). A hostname may be a DNS name or a
dotted IP address. If the hostname is omitted, the platform's
default behavior is used when binding the listening socket (''
is passed to socket.bind() as the hostname portion of the
address).
read-only
Flag indicating whether the server should operate in read-only
mode. Defaults to false. Note that even if the server is
operating in writable mode, individual storages may still be
read-only. But if the server is in read-only mode, no write
operations are allowed, even if the storages are writable. Note
that pack() is considered a read-only operation.
invalidation-queue-size
The storage server keeps a queue of the objects modified by the
last N transactions, where N == invalidation_queue_size. This
queue is used to speed client cache verification when a client
disconnects for a short period of time.
monitor-address
The address at which the monitor server should listen. If
specified, a monitor server is started. The monitor server
provides server statistics in a simple text format. This can
be in the form 'host:port' to signify a TCP/IP connection or a
pathname string to signify a Unix domain socket connection (at
least one '/' is required). A hostname may be a DNS name or a
dotted IP address. If the hostname is omitted, the platform's
default behavior is used when binding the listening socket (''
is passed to socket.bind() as the hostname portion of the
address).
transaction-timeout
The maximum amount of time to wait for a transaction to commit
after acquiring the storage lock, specified in seconds. If the
transaction takes too long, the client connection will be closed
and the transaction aborted.
authentication-protocol
The name of the protocol used for authentication. The
only protocol provided with ZEO is "digest," but extensions
may provide other protocols.
authentication-database
The path of the database containing authentication credentials.
authentication-realm
The authentication realm of the server. Some authentication
schemes use a realm to identify the logic set of usernames
that are accepted by this server.
Configuring client
------------------
The ZEO client can also be configured using ZConfig. The ZODB.config
module provides several function for opening a storage based on its
configuration.
- ZODB.config.storageFromString()
- ZODB.config.storageFromFile()
- ZODB.config.storageFromURL()
The ZEO client configuration requires the server address be
specified. Everything else is optional. An example configuration is::
<zeoclient>
server zeo.example.com:8090
</zeoclient>
To use a ZEO client from Zope, write a configuration file and load it
from custom_zodb.py::
from ZODB.config import storageFromURL
Storage = storageFromURL("/path/to/client.txt")
The other configuration options are listed below.
storage
The name of the storage that the client wants to use. If the
ZEO server serves more than one storage, the client selects
the storage it wants to use by name. The default name is '1',
which is also the default name for the ZEO server.
cache-size
The maximum size of the client cache, in bytes.
name
The storage name. If unspecified, the address of the server
will be used as the name.
client
Enables persistent cache files. The string passed here is
used to construct the cache filenames. If it is not
specified, the client creates a temporary cache that will
only be used by the current object.
var
The directory where persistent cache files are stored. By
default cache files, if they are persistent, are stored in
the current directory.
min-disconnect-poll
The minimum delay in seconds between attempts to connect to
the server, in seconds. Defaults to 5 seconds.
max-disconnect-poll
The maximum delay in seconds between attempts to connect to
the server, in seconds. Defaults to 300 seconds.
wait
A boolean indicating whether the constructor should wait
for the client to connect to the server and verify the cache
before returning. The default is true.
read-only
A flag indicating whether this should be a read-only storage,
defaulting to false (i.e. writing is allowed by default).
read-only-fallback
A flag indicating whether a read-only remote storage should be
acceptable as a fallback when no writable storages are
available. Defaults to false. At most one of read_only and
read_only_fallback should be true.
realm
The authentication realm of the server. Some authentication
schemes use a realm to identify the logic set of usernames
that are accepted by this server.
A ZEO client can also be created by calling the ClientStorage
constructor explicitly. For example::
from ZEO.ClientStorage import ClientStorage
storage = ClientStorage(("zeo.example.com", 8090))
Running the ZEO server as a daemon
----------------------------------
In an operational setting, you will want to run the ZEO server a
daemon process that is restarted when it dies. The zdaemon package
provides two tools for running daemons: zdrun.py and zdctl.py. The
document "Using zdctl and zdrun to manage server processes"
(Doc/zdctl.txt) explains how to use these scripts to manage daemons.
Rotating log files
~~~~~~~~~~~~~~~~~~
ZEO will re-initialize its logging subsystem when it receives a
SIGUSR2 signal. If you are using the standard event logger, you
should first rename the log file and then send the signal to the
server. The server will continue writing to the renamed log file
until it receives the signal. After it receives the signal, the
server will create a new file with the old name and write to it.
Tools
-----
There are a few scripts that may help running a ZEO server. The
zeopack.py script connects to a server and packs the storage. It can
be run as a cron job. The zeoup.py script attempts to connect to a
ZEO server and verify that is is functioning. The zeopasswd.py script
manages a ZEO servers password database.
Diagnosing problems
-------------------
If an exception occurs on the server, the server will log a traceback
and send an exception to the client. The traceback on the client will
show a ZEO protocol library as the source of the error. If you need
to diagnose the problem, you will have to look in the server log for
the rest of the traceback.
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ZEO Client Cache Tracing
========================
An important question for ZEO users is: how large should the ZEO
client cache be? ZEO 2 (as of ZEO 2.0b2) has a new feature that lets
you collect a trace of cache activity and tools to analyze this trace,
enabling you to make an informed decision about the cache size.
Don't confuse the ZEO client cache with the Zope object cache. The
ZEO client cache is only used when an object is not in the Zope object
cache; the ZEO client cache avoids roundtrips to the ZEO server.
Enabling Cache Tracing
----------------------
To enable cache tracing, set the environment variable ZEO_CACHE_TRACE
to the name of a file to which the ZEO client process can write. ZEO
will append a hyphen and the storage name to the filename, to
distinguish different storages. If the file doesn't exist, the ZEO
will try to create it. If there are problems with the file, a log
message is written to the standard Zope log file. To start or stop
tracing, the ZEO client process (typically a Zope application server)
must be restarted.
The trace file can grow pretty quickly; on a moderately loaded server,
we observed it growing by 5 MB per hour. The file consists of binary
records, each 24 bytes long; a detailed description of the record
lay-out is given in stats.py. No sensitive data is logged.
Analyzing a Cache Trace
-----------------------
The stats.py command-line tool is the first-line tool to analyze a
cache trace. Its default output consists of two parts: a one-line
summary of essential statistics for each segment of 15 minutes,
interspersed with lines indicating client restarts and "cache flip
events" (more about those later), followed by a more detailed summary
of overall statistics.
The most important statistic is probably the "hit rate", a percentage
indicating how many requests to load an object could be satisfied from
the cache. Hit rates around 70% are good. 90% is probably close to
the theoretical maximum. If you see a hit rate under 60% you can
probably improve the cache performance (and hence your Zope
application server's performance) by increasing the ZEO cache size.
This is normally configured using the cache_size keyword argument to
the ClientStorage() constructor in your custom_zodb.py file. The
default cache size is 20 MB.
The stats.py tool shows its command line syntax when invoked without
arguments. The tracefile argument can be a gzipped file if it has a
.gz extension. It will read from stdin (assuming uncompressed data)
if the tracefile argument is '-'.
Simulating Different Cache Sizes
--------------------------------
Based on a cache trace file, you can make a prediction of how well the
cache might do with a different cache size. The simul.py tool runs an
accurate simulation of the ZEO client cache implementation based upon
the events read from a trace file. A new simulation is started each
time the trace file records a client restart event; if a trace file
contains more than one restart event, a separate line is printed for
each simulation, and line with overall statistics is added at the end.
Example, assuming the trace file is in /tmp/cachetrace.log::
$ python simul.py -s 100 /tmp/cachetrace.log
START TIME DURATION LOADS HITS INVALS WRITES FLIPS HITRATE
Sep 4 11:59 38:01 59833 40473 257 20 2 67.6%
$
This shows that with a 100 MB cache size, the cache hit rate is
67.6%. So let's try this again with a 200 MB cache size::
$ python simul.py -s 200 /tmp/cachetrace.log
START TIME DURATION LOADS HITS INVALS WRITES FLIPS HITRATE
Sep 4 11:59 38:01 59833 40921 258 20 1 68.4%
$
This showed hardly any improvement. So let's try a 300 MB cache
size::
$ python2.0 simul.py -s 300 /tmp/cachetrace.log
ZEOCacheSimulation, cache size 300,000,000 bytes
START TIME DURATION LOADS HITS INVALS WRITES FLIPS HITRATE
Sep 4 11:59 38:01 59833 40921 258 20 0 68.4%
$
This shows that for this particular trace file, the maximum attainable
hit rate is 68.4%. This is probably caused by the fact that nearly a
third of the objects mentioned in the trace were loaded only once --
the cache only helps if an object is loaded more than once.
The simul.py tool also supports simulating different cache
strategies. Since none of these are implemented, these are not
further documented here.
Cache Flips
-----------
The cache uses two files, which are managed as follows:
- Data are written to file 0 until file 0 exceeds limit/2 in size.
- Data are written to file 1 until file 1 exceeds limit/2 in size.
- File 0 is truncated to size 0 (or deleted and recreated).
- Data are written to file 0 until file 0 exceeds limit/2 in size.
- File 1 is truncated to size 0 (or deleted and recreated).
- Data are written to file 1 until file 1 exceeds limit/2 in size.
and so on.
A switch from file 0 to file 1 is called a "cache flip". At all cache
flips except the first, half of the cache contents is wiped out. This
affects cache performance. How badly this impact is can be seen from
the per-15-minutes summaries printed by stats.py. The -i option lets
you choose a smaller summary interval which shows the impact more
acutely.
The simul.py tool shows the number of cache flips in the FLIPS column.
If you see more than one flip per hour the cache may be too small.
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