couchdb-cluster-admin 0.4.2

# couchdb-cluster-admin
utility for managing multi-node couchdb 2.x clusters

# First, put together a config file for your setup

This will make the rest of the commands simpler to run. Copy the example

```
cp config/conf.example.yml config/mycluster.yml
```

and then edit it with the details of your cluster.

# Setting up a local cluster to test on

If you have docker installed you can just run

```bash
docker build -t couchdb-cluster - < docker-couchdb-cluster/Dockerfile
```

to build the cluster image (based on klaemo/couchdb:2.0-dev) and then run

```bash
docker run --name couchdb-cluster \
-p 15984:15984 \
-p 15986:15986 \
-p 25984:25984 \
-p 25986:25986 \
-p 35984:35984 \
-p 35986:35986 \
-p 45984:45984 \
-p 45986:45986 \
-v $(pwd)/data:/usr/src/couchdb/dev/lib/ \
-t couchdb-cluster \
--with-admin-party-please \
-n 4
```

to start a cluster with 4 nodes. The nodes' data will be persisted to `./data`.

To run the tests (which require this docker setup), download and install https://github.com/sstephenson/bats

```bash
git clone https://github.com/sstephenson/bats.git
cd bats
./install.sh /usr/local # or wherever on your PATH you want to install this
```

and then

```bash
docker start couchdb-cluster # make sure this is running and localhost:15984 is receiving pings
bats test/
```

# Optional: Set password in environment

If you do not wish to specify your password every time you run a command,
you may put its value in the `COUCHDB_CLUSTER_ADMIN_PASSWORD` environment variable like so:

```
read -sp Password: PW
```

Then, for all commands below prefex the command with `COUCHDB_CLUSTER_ADMIN_PASSWORD=$PW`, e.g.

```
COUCHDB_CLUSTER_ADMIN_PASSWORD=$PW python couchdb-admin-cluster/describe.py --conf mycluster.yml
```

# Get a quick overview of your cluster

Now you can run

```
python couchdb_cluster_admin/describe.py --conf config/mycluster.yml
```

to see an overview of your cluster nodes and shard allocation.
For example, in the following output:

```
Membership
cluster_nodes: couch3 couch1 couch4 couch2
all_nodes: couch3 couch1 couch4 couch2
Shards
00000000-1fffffff 20000000-3fffffff 40000000-5fffffff 60000000-7fffffff 80000000-9fffffff a0000000-bfffffff c0000000-dfffffff e0000000-ffffffff
mydb couch1 couch1 couch1 couch1 couch1 couch1 couch1 couch1
my_second_database couch1 couch1 couch1 couch1 couch1 couch1 couch1 couch1
```

you can see that while there are four nodes,
all shards are currently assigned only to the first node.

# Help estimating shard allocation

In order to plan out a shard reallocation, you can run the following command:

```bash
python couchdb_cluster_admin/suggest_shard_allocation.py --conf config/mycluster.yml --allocate couch1:1 couch2,couch3,couch4:2
```

The values for the `--allocate` arg in the example above should be interpreted as
"Put 1 copy on couch1, and put 2 copies spread across couch2, couch3, and couch4".

The output looks like this:

```
couch1 57.57 GB
couch2 42.15 GB
couch3 36.5 GB
couch4 36.5 GB
00000000-1fffffff 20000000-3fffffff 40000000-5fffffff 60000000-7fffffff 80000000-9fffffff a0000000-bfffffff c0000000-dfffffff e0000000-ffffffff
mydb couch1,couch2,couch4 couch1,couch2,couch3 couch1,couch3,couch4 couch1,couch2,couch4 couch1,couch2,couch3 couch1,couch3,couch4 couch1,couch2,couch4 couch1,couch2,couch3
my_second_database couch1,couch3,couch4 couch1,couch3,couch4 couch1,couch3,couch4 couch1,couch3,couch4 couch1,couch3,couch4 couch1,couch3,couch4 couch1,couch3,couch4 couch1,couch3,couch4
```

Note, the reallocation does not take into account the current location of shards,
so it is much more useful in the situation that you're moving from a single-node cluster
to a multi-node cluster than it is in the situation where you're adding one more node to a multi-node cluster.
In the example above, couch1 would be the single-node cluster and couch2, couch3, and couch4
form are the multi-node cluster–to-be. You can imagine that after implementing
the shard allocation suggested here, we might remove all shards from couch1 and remove it from the cluster.

Note also that there is no guarantee that the "same" shard of different databases will go to the same node;
each (db, shard)-pair is treated as an independent unit when making computing an even shard allocation.
In this example there are only a few dbs and shards; when shards * dbs is high,
this process can be quite good at evenly balancing your data across nodes.