ZEO
Project description
ZEO provides a client-server storage implementation for ZODB.
Usage
ZEO is a client-server system for sharing a single storage among many clients. 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.
Options
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.
Connection management
A ZEO client manages its connection to the ZEO server. If it loses the connection, it attempts to reconnect. While it is disconnected, it can satisfy some reads by using its cache.
The client can be configured to wait for 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.
If a single address resolves to multiple IPv4 or IPv6 addresses, the client will connect to an arbitrary of these addresses.
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 installed like other Python packages using pip, easy_install, buildout, etc.
Configuring server
The script runzeo runs the ZEO server. The server can be configured using command-line arguments or a config file. This document only describes the config file. Run runzeo.py -h to see the list of command-line arguments.
The runzeo 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
</zeo>
<filestorage>
path /var/tmp/Data.fs
</filestorage>
<eventlog>
level INFO
<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. 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 runzeo -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.
- 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 clients
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>
The other configuration options are listed below.
- 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. You can find zdaemon and it’s documentation at http://pypi.python.org/pypi/zdaemon.
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 script connects to a server and packs the storage. It can be run as a cron job. 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.
Compatibility
ZEO 4.0.0 requires Python 2.6 or later.
- Note –
When using ZEO and upgrading from Python 2.4, you need to upgrade clients and servers at the same time, or upgrade clients first and then servers. Clients running Python 2.5 or 2.6 will work with servers running Python 2.4. Clients running Python 2.4 won’t work properly with servers running Python 2.5 or later due to changes in the way Python implements exceptions.
For a long time ZEO has been distributes with ZODB. ZEO 4 is is now maintained as a separate project.
ZEO clients from ZODB 3.2 on can talk to ZEO 4.0 servers. ZEO 4.0 clients talk to ZODB 3.8, 3.9, and 3.10 and ZEO 4.0 servers.
- Note –
ZEO 4.0 servers don’t support undo for clients older than ZODB 3.10.
Testing for downloaders
You can run the tests with:
python setup.py test
however, there’s an issue with getting the dependencies installed propely in a single run. If the first run fails installing dependencies, try running the above command a second time.
Testing for Developers
The ZEO checkouts are buildouts. When working from a ZODB checkout, first run the bootstrap.py script to initialize the buildout:
% python bootstrap.py
and then use the buildout script to build ZODB and gather the dependencies:
% bin/buildout
This creates a test script:
% bin/test -v
This command will run all the tests, printing a single dot for each test. When it finishes, it will print a test summary. The exact number of tests can vary depending on platform and available third-party libraries.:
Ran 1182 tests in 241.269s OK
The test script has many more options. Use the -h or --help options to see a file list of options. The default test suite omits several tests that depend on third-party software or that take a long time to run. To run all the available tests use the --all option. Running all the tests takes much longer.:
Ran 1561 tests in 1461.557s OK
More information
For more information on ZEO, see http://zodb.org
There is a Mailman mailing list in place to discuss all issues related to ZODB, including ZEO. You can send questions to
or subscribe at
and view its archives at
Note that Zope mailing lists have a subscriber-only posting policy.
Bugs and Patches
Bug reports and patches should be added to the Launchpad:
Changelog
4.2.1 (2016-06-30)
Fix bug connecting to localhost on Windows. (#8).
4.2.0 (2016-06-15)
Changed loadBefore to operate more like load behaved, especially with regard to the load lock. This allowes ZEO to work with the upcoming ZODB 5, which used loadbefore rather than load.
Reimplemented load using loadBefore, thus testing loadBefore extensively via existing tests.
Other changes to work with ZODB 5 (as well as ZODB 4)
Fixed: the ZEO cache loadBefore method failed to utilize current data.
Drop support for Python 2.6 and 3.2.
4.2.0b1 (2015-06-05)
Add support for PyPy.
4.1.0 (2015-01-06)
Add support for Python 3.4.
Added a new ruok client protocol for getting server status on the ZEO port without creating a full-blown client connection and without logging in the server log.
Log errors on server side even if using multi threaded delay.
4.0.0 (2013-08-18)
Avoid reading excess random bytes when setting up an auth_digest session.
Optimize socket address enumeration in ZEO client (avoid non-TCP types).
Improve Travis CI testing support.
Assign names to all threads for better runtime debugging.
Fix “assignment to keyword” error under Py3k in ‘ZEO.scripts.zeoqueue’.
4.0.0b1 (2013-05-20)
Depend on ZODB >= 4.0.0b2
Add support for Python 3.2 / 3.3.
4.0.0a1 (2012-11-19)
First (in a long time) separate ZEO release.
Since ZODB 3.10.5:
Storage servers now emit Serving and Closed events so subscribers can discover addresses when dynamic port assignment (bind to port 0) is used. This could, for example, be used to update address information in a ZooKeeper database.
Client storages have a method, new_addr, that can be used to change the server address(es). This can be used, for example, to update a dynamically determined server address from information in a ZooKeeper database.
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