Expiring Object Support

Expiring Object Support

The swift-object-expirer offers scheduled deletion of objects. The Swift client would use the X-Delete-At or X-Delete-After headers during an object PUT or POST and the cluster would automatically quit serving that object at the specified time and would shortly thereafter remove the object from the system.

The X-Delete-At header takes a Unix Epoch timestamp, in integer form; for example: 1317070737 represents Mon Sep 26 20:58:57 2011 UTC.

The X-Delete-After header takes a positive integer number of seconds. The proxy server that receives the request will convert this header into an X-Delete-At header using the request timestamp plus the value given.

If both the X-Delete-At and X-Delete-After headers are sent with a request then the X-Delete-After header will take precedence.

As expiring objects are added to the system, the object servers will record the expirations in a hidden .expiring_objects account for the swift-object-expirer to handle later.

Usually, just one instance of the swift-object-expirer daemon needs to run for a cluster. This isn’t exactly automatic failover high availability, but if this daemon doesn’t run for a few hours it should not be any real issue. The expired-but-not-yet-deleted objects will still 404 Not Found if someone tries to GET or HEAD them and they’ll just be deleted a bit later when the daemon is restarted.

By default, the swift-object-expirer daemon will run with a concurrency of 1. Increase this value to get more concurrency. A concurrency of 1 may not be enough to delete expiring objects in a timely fashion for a particular Swift cluster.

It is possible to run multiple daemons to do different parts of the work if a single process with a concurrency of more than 1 is not enough (see the sample config file for details).

To run the swift-object-expirer as multiple processes, set processes to the number of processes (either in the config file or on the command line). Then run one process for each part. Use process to specify the part of the work to be done by a process using the command line or the config. So, for example, if you’d like to run three processes, set processes to 3 and run three processes with process set to 0, 1, and 2 for the three processes. If multiple processes are used, it’s necessary to run one for each part of the work or that part of the work will not be done.

By default the daemon looks for two different config files. When launching, the process searches for the [object-expirer] section in the

/etc/swift/object-server.conf config. If the section or the config is missing it will then look for and use the /etc/swift/object-expirer.conf config. The latter config file is considered deprecated and is searched for to aid in cluster upgrades.

Upgrading impact: General Task Queue vs Legacy Queue

The expirer daemon will be moving to a new general task-queue based design that will divide the work across all object servers, as such only expirers defined in the object-server config will be able to use the new system. The parameters in both files are identical except for a new option in the object-server [object-expirer] section, dequeue_from_legacy_queue which when set to True will tell the expirer that in addition to using the new task queueing system to also check the legacy (soon to be deprecated) queue.

Note

The new task-queue system has not been completed yet. So an expirer’s with dequeue_from_legacy_queue set to False will currently do nothing.

By default dequeue_from_legacy_queue will be False, it is necessary to be set to True explicitly while migrating from the old expiring queue.

Any expirer using the old config /etc/swift/object-expirer.conf will not use the new general task queue. It’ll ignore the dequeue_from_legacy_queue and will only check the legacy queue. Meaning it’ll run as a legacy expirer.

Why is this important? If you are currently running object-expirers on nodes that are not object storage nodes, then for the time being they will still work but only by dequeuing from the old queue. When the new general task queue is introduced, expirers will be required to run on the object servers so that any new objects added can be removed. If you’re in this situation, you can safely setup the new expirer section in the object-server.conf to deal with the new queue and leave the legacy expirers running elsewhere.

However, if your old expirers are running on the object-servers, the most common topology, then you would add the new section to all object servers, to deal the new queue. In order to maintain the same number of expirers checking the legacy queue, pick the same number of nodes as you previously had and turn on dequeue_from_legacy_queue on those nodes only. Also note on these nodes you’d need to keep the legacy process and processes options to maintain the concurrency level for the legacy queue.

Note

Be careful not to enable dequeue_from_legacy on too many expirers as all legacy tasks are stored in a single hidden account and the same hidden containers. On a large cluster one may inadvertently overload the acccount/container servers handling the legacy expirer queue.

Here is a quick sample of the object-expirer section required in the object-server.conf:

[object-expirer]
# log_name = object-expirer
# log_facility = LOG_LOCAL0
# log_level = INFO
# log_address = /dev/log
#
interval = 300

# If this true, expirer execute tasks in legacy expirer task queue
dequeue_from_legacy_queue = false

# processes can only be used in conjunction with `dequeue_from_legacy_queue`.
# So this option is ignored if dequeue_from_legacy_queue=false.
# processes is how many parts to divide the legacy work into, one part per
# process that will be doing the work
# processes set 0 means that a single legacy process will be doing all the work
# processes can also be specified on the command line and will override the
# config value
# processes = 0

# process can only be used in conjunction with `dequeue_from_legacy_queue`.
# So this option is ignored if dequeue_from_legacy_queue=false.
# process is which of the parts a particular legacy process will work on
# process can also be specified on the command line and will override the config
# value
# process is "zero based", if you want to use 3 processes, you should run
# processes with process set to 0, 1, and 2
# process = 0

report_interval = 300

# request_tries is the number of times the expirer's internal client will
# attempt any given request in the event of failure. The default is 3.
# request_tries = 3

# concurrency is the level of concurrency to use to do the work, this value
# must be set to at least 1
# concurrency = 1

# The expirer will re-attempt expiring if the source object is not available
# up to reclaim_age seconds before it gives up and deletes the entry in the
# queue.
# reclaim_age = 604800

And for completeness, here is a quick sample of the legacy object-expirer.conf file:

[DEFAULT]
# swift_dir = /etc/swift
# user = swift
# You can specify default log routing here if you want:
# log_name = swift
# log_facility = LOG_LOCAL0
# log_level = INFO

[object-expirer]
interval = 300

[pipeline:main]
pipeline = catch_errors cache proxy-server

[app:proxy-server]
use = egg:swift#proxy
# See proxy-server.conf-sample for options

[filter:cache]
use = egg:swift#memcache
# See proxy-server.conf-sample for options

[filter:catch_errors]
use = egg:swift#catch_errors
# See proxy-server.conf-sample for options

Note

When running legacy expirers, the daemon needs to run on a machine with access to all the backend servers in the cluster, but does not need proxy server or public access. The daemon will use its own internal proxy code instance to access the backend servers.

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