[3/3] kudu git commit: docs: design for handling permanent master failures

[to...@apache.org]

docs: design for handling permanent master failures

Here's a design doc that describes how we might address permanent master
failures. The downside of the proposed solution is that it requires DNS
manipulation, but the upside is that it can be adapted to migrate single
node deployments to multiple masters.

Change-Id: I2f05c319c89cf37e2d71fdc4b7ec951b2932a2b2
Reviewed-on: http://gerrit.cloudera.org:8080/3393
Tested-by: Kudu Jenkins
Reviewed-by: Todd Lipcon <to...@apache.org>


Project: http://git-wip-us.apache.org/repos/asf/kudu/repo
Commit: http://git-wip-us.apache.org/repos/asf/kudu/commit/13ea2802
Tree: http://git-wip-us.apache.org/repos/asf/kudu/tree/13ea2802
Diff: http://git-wip-us.apache.org/repos/asf/kudu/diff/13ea2802

Branch: refs/heads/master
Commit: 13ea28029773c042c77f6a0325e21c30f298bb62
Parents: 07c50b8
Author: Adar Dembo <ad...@cloudera.com>
Authored: Thu Jun 16 11:32:02 2016 -0700
Committer: Todd Lipcon <to...@apache.org>
Committed: Wed Aug 17 23:02:04 2016 +0000

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 docs/design-docs/README.md                  |   1 +
 docs/design-docs/master-perm-failure-1.0.md | 111 +++++++++++++++++++++++
 2 files changed, 112 insertions(+)
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http://git-wip-us.apache.org/repos/asf/kudu/blob/13ea2802/docs/design-docs/README.md
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diff --git a/docs/design-docs/README.md b/docs/design-docs/README.md
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@@ -39,3 +39,4 @@ made.
 | [Scan Token API](scan-tokens.md) | Client | [gerrit](http://gerrit.cloudera.org:8080/2443) |
 | [Full multi-master support for Kudu 1.0](multi-master-1.0.md) | Master, Client | [gerrit](http://gerrit.cloudera.org:8080/2527) |
 | [Non-covering Range Partitions](non-covering-range-partitions.md) | Master, Client | [gerrit](http://gerrit.cloudera.org:8080/2772) |
+| [Permanent failure handling of masters for Kudu 1.0](master-perm-failure-1.0.md) | Master | |

http://git-wip-us.apache.org/repos/asf/kudu/blob/13ea2802/docs/design-docs/master-perm-failure-1.0.md
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+<!---
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+
+# Permanent failure handling of masters for Kudu 1.0
+
+## Background
+
+Kudu's 1.0 release includes various improvements to multi-master support so that
+it can be used in production safely. The original release plan emphasized using
+multiple masters for high availability in the event of transient failures, but
+unfortunately didn't talk much (if at all) about permanent failures. This
+document compares transient and permanent failures, and provides a design for
+addressing the latter.
+
+### Transient failures
+
+Kudu's handling of transient master failures is best illustrated with an
+example. Assume we have a healthy Raft configuration consisting of three
+masters. If one master suffers a transient failure and is offline for a short
+time before returning, there's no harm. If the failed node was a follower, the
+leader can still replicate to a majority of nodes. If the leader itself failed,
+a majority of nodes can still elect a new leader. No new machinery is needed to
+support this; all of the code has been written and tested with the caveat that
+there are some bugs that we have been squashing over the past few months.
+
+### Permanent failures
+
+What's missing, however, is handling for permanent failures. If a node dies and
+is not coming back, we need to replace it with a healthy one ASAP. If we don't,
+a second failure (transient or otherwise) will lead to a loss of availability.
+
+## Design proposal for handling permanent failures
+
+In practice, the most straight-forward approach to handling any permanent
+failure is to extend Raft configuration change support to the master; currently
+it\u2019s only possible to do it in the tserver. However, this just isn't possible
+given time constraints. Therefore, we will use a DNS-dependent alternative.
+
+Here is the algorithm:
+
+1. Base state:
+   1. There's a healthy Raft configuration of three nodes: **A**, **B**, and
+      **C**.
+   2. **A** is the leader.
+   3. The value of **--master_addresses** (the master-only gflag describing the
+      locations of the entire master Raft configuration) is {**A**, **B**,
+      **C**} on each node.
+   4. Each of **A**, **B**, and **C** are DNS cnames.
+   5. The value of **--tserver_master_address** (the tserver-only gflag
+      describing the locations of the masters) on each tserver is {**A**, **B**,
+      **C**}
+2. **C** dies, permanently. If **A** dies, the directions below are the same,
+   except replace **A** with whichever node was elected the new leader.
+3. Make sure **C** is completely dead and cannot come back to life. If possible,
+   destroy its on-disk master state.
+4. Find a replacement machine **D**.
+5. Modify DNS records such that **D** assumes **C**'s cname.
+6. Invoke new command line tool on **D** that uses remote bootstrap to copy
+   master state from **A** to **D**.
+7. Start a new master on **D**. It should use the same value of
+   **--master_addresses** as used by the other masters.
+
+In order to implement this design, we'll need to make the following changes:
+
+1. Make remote bootstrap available for masters (currently it's tserver-only).
+2. Implement new remote bootstrap "client" command line tool.
+
+## Migration from single-master deployments
+
+While not exactly related to failure handling, the remote bootstrap
+modifications described above can be used to ease migration from a single master
+deployment to a multi-master one. Since migration is a rare and singular event
+in the lifetime of a cluster, it is assumed that a temporary loss of
+availability during the migration is acceptable.
+
+Here is the algorithm:
+
+1. There exists a healthy single-node master deployment called **A**.
+2. Find new master machines, creating DNS cnames for all of them. Create a DNS
+   cname for **A** too, if it's not already a cname. Note: the total number of
+   masters must be odd. To figure out how many masters there should be, consider
+   that **N** failures can be tolerated by a deployment of **2N+1** masters.
+3. Stop the master running on **A**.
+4. Invoke new command line tool to format a filesystem on each new master node.
+5. Invoke new command line tool to print the filesystem uuid on each master node
+   and on existing master node **A**. Record these UUIDs.
+6. Invoke new command line tool on **A** to rewrite the on-disk consensus
+   metadata (cmeta) file describing the Raft configuration. Provide the uuid and
+   cname for each new master node as well as for **A**.
+7. Start the master running on **A**.
+8. Invoke remote bootstrap "client" tool from above on each new node to copy
+   **A**'s master state onto new node. These invocations can be done in parallel
+   to speed up the process, though in practice master state is quite small.
+9. Start the master on each new node.
+
+In order to implement this design, we'll need the following additional changes:
+
+1. Implement new command line tool to format filesystems.
+2. Implement new command line tool to print filesystem uuids.
+3. Implement new command line tool to rewrite cmeta files.