High Availability and PyMongo¶
PyMongo makes it easy to write highly available applications whether you use a single replica set or a large sharded cluster.
Connecting to a Replica Set¶
PyMongo makes working with replica sets easy. Here we’ll launch a new replica set and show how to handle both initialization and normal connections with PyMongo.
Note
Replica sets require server version >= 1.6.0. Support for connecting to replica sets also requires PyMongo version >= 1.8.0.
Starting a Replica Set¶
The main replica set documentation contains extensive information about setting up a new replica set or migrating an existing MongoDB setup, be sure to check that out. Here, we’ll just do the bare minimum to get a three node replica set setup locally.
Warning
Replica sets should always use multiple nodes in production - putting all set members on the same physical node is only recommended for testing and development.
We start three mongod
processes, each on a different port and with
a different dbpath, but all using the same replica set name “foo”. In
the example we use the hostname “morton.local”, so replace that with
your hostname when running:
$ hostname
morton.local
$ mongod --replSet foo/morton.local:27018,morton.local:27019 --rest
$ mongod --port 27018 --dbpath /data/db1 --replSet foo/morton.local:27017 --rest
$ mongod --port 27019 --dbpath /data/db2 --replSet foo/morton.local:27017 --rest
Initializing the Set¶
At this point all of our nodes are up and running, but the set has yet to be initialized. Until the set is initialized no node will become the primary, and things are essentially “offline”.
To initialize the set we need to connect to a single node and run the initiate command. Since we don’t have a primary yet, we’ll need to tell PyMongo that it’s okay to connect to a slave/secondary:
>>> from pymongo import MongoClient, ReadPreference
>>> c = MongoClient("morton.local:27017",
read_preference=ReadPreference.SECONDARY)
Note
We could have connected to any of the other nodes instead, but only the node we initiate from is allowed to contain any initial data.
After connecting, we run the initiate command to get things started (here we just use an implicit configuration, for more advanced configuration options see the replica set documentation):
>>> c.admin.command("replSetInitiate")
{u'info': u'Config now saved locally. Should come online in about a minute.',
u'info2': u'no configuration explicitly specified -- making one', u'ok': 1.0}
The three mongod
servers we started earlier will now coordinate
and come online as a replica set.
Connecting to a Replica Set¶
The initial connection as made above is a special case for an
uninitialized replica set. Normally we’ll want to connect
differently. A connection to a replica set can be made using the
normal MongoClient()
constructor, specifying
one or more members of the set. For example, any of the following
will create a connection to the set we just created:
>>> MongoClient("morton.local", replicaset='foo')
MongoClient([u'morton.local:27019', 'morton.local:27017', u'morton.local:27018'])
>>> MongoClient("morton.local:27018", replicaset='foo')
MongoClient([u'morton.local:27019', u'morton.local:27017', 'morton.local:27018'])
>>> MongoClient("morton.local", 27019, replicaset='foo')
MongoClient(['morton.local:27019', u'morton.local:27017', u'morton.local:27018'])
>>> MongoClient(["morton.local:27018", "morton.local:27019"])
MongoClient(['morton.local:27019', u'morton.local:27017', 'morton.local:27018'])
>>> MongoClient("mongodb://morton.local:27017,morton.local:27018,morton.local:27019")
MongoClient(['morton.local:27019', 'morton.local:27017', 'morton.local:27018'])
The nodes passed to MongoClient()
are called
the seeds. If only one host is specified the replicaset parameter
must be used to indicate this isn’t a connection to a single node.
As long as at least one of the seeds is online, the driver will be able
to “discover” all of the nodes in the set and make a connection to the
current primary.
Handling Failover¶
When a failover occurs, PyMongo will automatically attempt to find the new primary node and perform subsequent operations on that node. This can’t happen completely transparently, however. Here we’ll perform an example failover to illustrate how everything behaves. First, we’ll connect to the replica set and perform a couple of basic operations:
>>> db = MongoClient("morton.local", replicaSet='foo').test
>>> db.test.save({"x": 1})
ObjectId('...')
>>> db.test.find_one()
{u'x': 1, u'_id': ObjectId('...')}
By checking the host and port, we can see that we’re connected to morton.local:27017, which is the current primary:
>>> db.connection.host
'morton.local'
>>> db.connection.port
27017
Now let’s bring down that node and see what happens when we run our query again:
>>> db.test.find_one()
Traceback (most recent call last):
pymongo.errors.AutoReconnect: ...
We get an AutoReconnect
exception. This means
that the driver was not able to connect to the old primary (which
makes sense, as we killed the server), but that it will attempt to
automatically reconnect on subsequent operations. When this exception
is raised our application code needs to decide whether to retry the
operation or to simply continue, accepting the fact that the operation
might have failed.
On subsequent attempts to run the query we might continue to see this exception. Eventually, however, the replica set will failover and elect a new primary (this should take a couple of seconds in general). At that point the driver will connect to the new primary and the operation will succeed:
>>> db.test.find_one()
{u'x': 1, u'_id': ObjectId('...')}
>>> db.connection.host
'morton.local'
>>> db.connection.port
27018
MongoReplicaSetClient¶
Using a MongoReplicaSetClient
instead
of a simple MongoClient
offers two key features:
secondary reads and replica set health monitoring. To connect using
MongoReplicaSetClient
just provide a
host:port pair and the name of the replica set:
>>> from pymongo import MongoReplicaSetClient
>>> MongoReplicaSetClient("morton.local:27017", replicaSet='foo')
MongoReplicaSetClient([u'morton.local:27019', u'morton.local:27017', u'morton.local:27018'])
Secondary Reads¶
By default an instance of MongoReplicaSetClient will only send queries to
the primary member of the replica set. To use secondaries for queries
we have to change the ReadPreference
:
>>> db = MongoReplicaSetClient("morton.local:27017", replicaSet='foo').test
>>> from pymongo.read_preferences import ReadPreference
>>> db.read_preference = ReadPreference.SECONDARY_PREFERRED
Now all queries will be sent to the secondary members of the set. If there are
no secondary members the primary will be used as a fallback. If you have
queries you would prefer to never send to the primary you can specify that
using the SECONDARY
read preference:
>>> db.read_preference = ReadPreference.SECONDARY
Read preference can be set on a client, database, collection, or on a per-query basis, e.g.:
>>> db.collection.find_one(read_preference=ReadPreference.PRIMARY)
Reads are configured using three options: read_preference, tag_sets, and secondary_acceptable_latency_ms.
read_preference:
PRIMARY
: Read from the primary. This is the default, and provides the strongest consistency. If no primary is available, raiseAutoReconnect
.PRIMARY_PREFERRED
: Read from the primary if available, or if there is none, read from a secondary matching your choice oftag_sets
andsecondary_acceptable_latency_ms
.SECONDARY
: Read from a secondary matching your choice oftag_sets
andsecondary_acceptable_latency_ms
. If no matching secondary is available, raiseAutoReconnect
.SECONDARY_PREFERRED
: Read from a secondary matching your choice oftag_sets
andsecondary_acceptable_latency_ms
if available, otherwise from primary (regardless of the primary’s tags and latency).NEAREST
: Read from any member matching your choice oftag_sets
andsecondary_acceptable_latency_ms
.
tag_sets:
Replica-set members can be tagged according to any
criteria you choose. By default, MongoReplicaSetClient ignores tags when
choosing a member to read from, but it can be configured with the tag_sets
parameter. tag_sets
must be a list of dictionaries, each dict providing tag
values that the replica set member must match. MongoReplicaSetClient tries each
set of tags in turn until it finds a set of tags with at least one matching
member. For example, to prefer reads from the New York data center, but fall
back to the San Francisco data center, tag your replica set members according
to their location and create a MongoReplicaSetClient like so:
>>> rsc = MongoReplicaSetClient(
... "morton.local:27017",
... replicaSet='foo'
... read_preference=ReadPreference.SECONDARY,
... tag_sets=[{'dc': 'ny'}, {'dc': 'sf'}]
... )
MongoReplicaSetClient tries to find secondaries in New York, then San Francisco,
and raises AutoReconnect
if none are available. As an
additional fallback, specify a final, empty tag set, {}
, which means “read
from any member that matches the mode, ignoring tags.”
secondary_acceptable_latency_ms:
If multiple members match the mode and tag sets, MongoReplicaSetClient reads
from among the nearest members, chosen according to ping time. By default,
only members whose ping times are within 15 milliseconds of the nearest
are used for queries. You can choose to distribute reads among members with
higher latencies by setting secondary_acceptable_latency_ms
to a larger
number. In that case, MongoReplicaSetClient distributes reads among matching
members within secondary_acceptable_latency_ms
of the closest member’s
ping time.
Note
secondary_acceptable_latency_ms
is ignored when talking to a
replica set through a mongos. The equivalent is the localThreshold command
line option.
Health Monitoring¶
When MongoReplicaSetClient is initialized it launches a background task to monitor the replica set for changes in:
- Health: detect when a member goes down or comes up, or if a different member becomes primary
- Configuration: detect changes in tags
- Latency: track a moving average of each member’s ping time
Replica-set monitoring ensures queries are continually routed to the proper members as the state of the replica set changes.
It is critical to call
close()
to terminate
the monitoring task before your process exits.
High Availability and mongos¶
An instance of MongoClient
can be configured
to automatically connect to a different mongos if the instance it is
currently connected to fails. If a failure occurs, PyMongo will attempt
to find the nearest mongos to perform subsequent operations. As with a
replica set this can’t happen completely transparently, Here we’ll perform
an example failover to illustrate how everything behaves. First, we’ll
connect to a sharded cluster, using a seed list, and perform a couple of
basic operations:
>>> db = MongoClient('morton.local:30000,morton.local:30001,morton.local:30002').test
>>> db.test.save({"x": 1})
ObjectId('...')
>>> db.test.find_one()
{u'x': 1, u'_id': ObjectId('...')}
Each member of the seed list passed to MongoClient must be a mongos. By checking the host, port, and is_mongos attributes we can see that we’re connected to morton.local:30001, a mongos:
>>> db.connection.host
'morton.local'
>>> db.connection.port
30001
>>> db.connection.is_mongos
True
Now let’s shut down that mongos instance and see what happens when we run our query again:
>>> db.test.find_one()
Traceback (most recent call last):
pymongo.errors.AutoReconnect: ...
As in the replica set example earlier in this document, we get
an AutoReconnect
exception. This means
that the driver was not able to connect to the original mongos at port
30001 (which makes sense, since we shut it down), but that it will
attempt to connect to a new mongos on subsequent operations. When this
exception is raised our application code needs to decide whether to retry
the operation or to simply continue, accepting the fact that the operation
might have failed.
As long as one of the seed list members is still available the next operation will succeed:
>>> db.test.find_one()
{u'x': 1, u'_id': ObjectId('...')}
>>> db.connection.host
'morton.local'
>>> db.connection.port
30002
>>> db.connection.is_mongos
True