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Container to Container Synchronization

Overview

Swift has a feature where all the contents of a container can be mirrored to another container through background synchronization. Swift cluster operators configure their cluster to allow/accept sync requests to/from other clusters, and the user specifies where to sync their container to along with a secret synchronization key.

Note

Container sync will sync object POSTs only if the proxy server is set to use "object_post_as_copy = true" which is the default. So-called fast object posts, "object_post_as_copy = false" do not update the container listings and therefore can't be detected for synchronization.

Note

If you are using the large objects feature you will need to ensure both your manifest file and your segment files are synced if they happen to be in different containers.

Configuring a Cluster's Allowable Sync Hosts

The Swift cluster operator must allow synchronization with a set of hosts before the user can enable container synchronization. First, the backend container server needs to be given this list of hosts in the container-server.conf file:

[DEFAULT]
# This is a comma separated list of hosts allowed in the
# X-Container-Sync-To field for containers.
# allowed_sync_hosts = 127.0.0.1
allowed_sync_hosts = host1,host2,etc.
...

[container-sync]
# You can override the default log routing for this app here (don't
# use set!):
# log_name = container-sync
# log_facility = LOG_LOCAL0
# log_level = INFO
# Will sync, at most, each container once per interval
# interval = 300
# Maximum amount of time to spend syncing each container
# container_time = 60

Tracking sync progress, problems, and just general activity can only be achieved with log processing for this first release of container synchronization. In that light, you may wish to set the above log_ options to direct the container-sync logs to a different file for easier monitoring. Additionally, it should be noted there is no way for an end user to detect sync progress or problems other than HEADing both containers and comparing the overall information.

Using the swift tool to set up synchronized containers

Note

The swift tool is available from the python-swiftclient library.

Note

You must be the account admin on the account to set synchronization targets and keys.

You simply tell each container where to sync to and give it a secret synchronization key. First, let's get the account details for our two cluster accounts:

$ swift -A http://cluster1/auth/v1.0 -U test:tester -K testing stat -v
StorageURL: http://cluster1/v1/AUTH_208d1854-e475-4500-b315-81de645d060e
Auth Token: AUTH_tkd5359e46ff9e419fa193dbd367f3cd19
   Account: AUTH_208d1854-e475-4500-b315-81de645d060e
Containers: 0
   Objects: 0
     Bytes: 0

$ swift -A http://cluster2/auth/v1.0 -U test2:tester2 -K testing2 stat -v
StorageURL: http://cluster2/v1/AUTH_33cdcad8-09fb-4940-90da-0f00cbf21c7c
Auth Token: AUTH_tk816a1aaf403c49adb92ecfca2f88e430
   Account: AUTH_33cdcad8-09fb-4940-90da-0f00cbf21c7c
Containers: 0
   Objects: 0
     Bytes: 0

Now, let's make our first container and tell it to synchronize to a second we'll make next:

$ swift -A http://cluster1/auth/v1.0 -U test:tester -K testing post \
  -t 'http://cluster2/v1/AUTH_33cdcad8-09fb-4940-90da-0f00cbf21c7c/container2' \
  -k 'secret' container1

The -t indicates the URL to sync to, which is the StorageURL from cluster2 we retrieved above plus the container name. The -k specifies the secret key the two containers will share for synchronization. Now, we'll do something similar for the second cluster's container:

$ swift -A http://cluster2/auth/v1.0 -U test2:tester2 -K testing2 post \
  -t 'http://cluster1/v1/AUTH_208d1854-e475-4500-b315-81de645d060e/container1' \
  -k 'secret' container2

That's it. Now we can upload a bunch of stuff to the first container and watch as it gets synchronized over to the second:

$ swift -A http://cluster1/auth/v1.0 -U test:tester -K testing \
  upload container1 .
photo002.png
photo004.png
photo001.png
photo003.png

$ swift -A http://cluster2/auth/v1.0 -U test2:tester2 -K testing2 \
  list container2

[Nothing there yet, so we wait a bit...]
[If you're an operator running SAIO and just testing, you may need to
 run 'swift-init container-sync once' to perform a sync scan.]

$ swift -A http://cluster2/auth/v1.0 -U test2:tester2 -K testing2 \
  list container2
photo001.png
photo002.png
photo003.png
photo004.png

You can also set up a chain of synced containers if you want more than two. You'd point 1 -> 2, then 2 -> 3, and finally 3 -> 1 for three containers. They'd all need to share the same secret synchronization key.

Using curl (or other tools) instead

So what's swift doing behind the scenes? Nothing overly complicated. It translates the -t <value> option into an X-Container-Sync-To: <value> header and the -k <value> option into an X-Container-Sync-Key: <value> header.

For instance, when we created the first container above and told it to synchronize to the second, we could have used this curl command:

$ curl -i -X POST -H 'X-Auth-Token: AUTH_tkd5359e46ff9e419fa193dbd367f3cd19' \
  -H 'X-Container-Sync-To: http://cluster2/v1/AUTH_33cdcad8-09fb-4940-90da-0f00cbf21c7c/container2' \
  -H 'X-Container-Sync-Key: secret' \
  'http://cluster1/v1/AUTH_208d1854-e475-4500-b315-81de645d060e/container1'
HTTP/1.1 204 No Content
Content-Length: 0
Content-Type: text/plain; charset=UTF-8
Date: Thu, 24 Feb 2011 22:39:14 GMT

What's going on behind the scenes, in the cluster?

The swift-container-sync does the job of sending updates to the remote container.

This is done by scanning the local devices for container databases and checking for x-container-sync-to and x-container-sync-key metadata values. If they exist, newer rows since the last sync will trigger PUTs or DELETEs to the other container.

Note

Container sync will sync object POSTs only if the proxy server is set to use "object_post_as_copy = true" which is the default. So-called fast object posts, "object_post_as_copy = false" do not update the container listings and therefore can't be detected for synchronization.

The actual syncing is slightly more complicated to make use of the three (or number-of-replicas) main nodes for a container without each trying to do the exact same work but also without missing work if one node happens to be down.

Two sync points are kept per container database. All rows between the two sync points trigger updates. Any rows newer than both sync points cause updates depending on the node's position for the container (primary nodes do one third, etc. depending on the replica count of course). After a sync run, the first sync point is set to the newest ROWID known and the second sync point is set to newest ROWID for which all updates have been sent.

An example may help. Assume replica count is 3 and perfectly matching ROWIDs starting at 1.

First sync run, database has 6 rows:

  • SyncPoint1 starts as -1.
  • SyncPoint2 starts as -1.
  • No rows between points, so no "all updates" rows.
  • Six rows newer than SyncPoint1, so a third of the rows are sent by node 1, another third by node 2, remaining third by node 3.
  • SyncPoint1 is set as 6 (the newest ROWID known).
  • SyncPoint2 is left as -1 since no "all updates" rows were synced.

Next sync run, database has 12 rows:

  • SyncPoint1 starts as 6.
  • SyncPoint2 starts as -1.
  • The rows between -1 and 6 all trigger updates (most of which should short-circuit on the remote end as having already been done).
  • Six more rows newer than SyncPoint1, so a third of the rows are sent by node 1, another third by node 2, remaining third by node 3.
  • SyncPoint1 is set as 12 (the newest ROWID known).
  • SyncPoint2 is set as 6 (the newest "all updates" ROWID).

In this way, under normal circumstances each node sends its share of updates each run and just sends a batch of older updates to ensure nothing was missed.