swift/doc/source/overview_container_sync.rst

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

The swift-container-sync process runs on each container server in the cluster and talks to the proxy servers in the remote cluster. Therefore, the container servers must be permitted to initiate outbound connections to the remote proxy servers.

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 in each container database. When syncing a container, the container-sync process figures out which replica of the container it has. In a standard 3-replica scenario, the process will have either replica number 0, 1, or 2. This is used to figure out which rows are belong to this sync process and which ones don't.

An example may help. Assume a replica count of 3 and database row IDs are 1..6. Also, assume that container-sync is running on this container for the first time, hence SP1 = SP2 = -1. :

SP1
SP2
 |
 v
-1 0 1 2 3 4 5 6

First, the container-sync process looks for rows with id between SP1 and SP2. Since this is the first run, SP1 = SP2 = -1, and there aren't any such rows. :

SP1
SP2
 |
 v
-1 0 1 2 3 4 5 6

Second, the container-sync process looks for rows with id greater than SP1, and syncs those rows which it owns. Ownership is based on the hash of the object name, so it's not always guaranteed to be exactly one out of every three rows, but it usually gets close. For the sake of example, let's say that this process ends up owning rows 2 and 5.

Once it's finished trying to sync those rows, it updates SP1 to be the biggest row-id that it's seen, which is 6 in this example. :

SP2           SP1
 |             |
 v             v
-1 0 1 2 3 4 5 6

While all that was going on, clients uploaded new objects into the container, creating new rows in the database. :

SP2           SP1
 |             |
 v             v
-1 0 1 2 3 4 5 6 7 8 9 10 11 12

On the next run, the container-sync starts off looking at rows with ids between SP1 and SP2. This time, there are a bunch of them. The sync process try to sync all of them. If it succeeds, it will set SP2 to equal SP1. If it fails, it will set SP2 to the failed object and will continue to try all other objects till SP1, setting SP2 to the first object that failed.

Under normal circumstances, the container-sync processes will have already taken care of synchronizing all rows, between SP1 and SP2, resulting in a set of quick checks. However, if one of the sync processes failed for some reason, then this is a vital fallback to make sure all the objects in the container get synchronized. Without this seemingly-redundant work, any container-sync failure results in unsynchronized objects. Note that the container sync will persistently retry to sync any faulty object until success, while logging each failure.

Once it's done with the fallback rows, and assuming no faults occured, SP2 is advanced to SP1. :

SP2
SP1
 |
 v
-1 0 1 2 3 4 5 6 7 8 9 10 11 12

Then, rows with row ID greater than SP1 are synchronized (provided this container-sync process is responsible for them), and SP1 is moved up to the greatest row ID seen. :

SP2            SP1
 |              |
 v              v
-1 0 1 2 3 4 5 6 7 8 9 10 11 12