Downgrade 4.0 Sharded Cluster to 3.6
Before you attempt any downgrade, familiarize yourself with the content of this document.
Downgrade Path
Once upgraded to 4.0, if you need to downgrade, we recommend downgrading to the latest patch release of 3.6.
Change Stream Consideration
MongoDB 4.0 introduces new hex-encoded string change stream resume tokens:
The resume token _data
type depends on the MongoDB versions and,
in some cases, the feature compatibility version (fcv) at the time
of the change stream's opening/resumption (i.e. a change in fcv
value does not affect the resume tokens for already opened change
streams):
MongoDB Version | Feature Compatibility Version | Resume Token _data Type |
---|---|---|
MongoDB 4.0.7 and later | "4.0" or "3.6" | Hex-encoded string ( v1 ) |
MongoDB 4.0.6 and earlier | "4.0" | Hex-encoded string ( v0 ) |
MongoDB 4.0.6 and earlier | "3.6" | BinData |
MongoDB 3.6 | "3.6" | BinData |
When downgrading from MongoDB 4.0.7 or greater, clients cannot use the resume tokens returned from the 4.0.7+ deployment. To resume a change stream, clients will need to use a pre-4.0.7 upgrade resume token (if available). Otherwise, clients will need to start a new change stream.
When downgrading from MongoDB 4.0.6 or earlier, clients can use BinData resume tokens returned from the 4.0 deployment, but not the
v0
tokens.
Create Backup
Optional but Recommended. Create a backup of your database.
Prerequisites
Before downgrading the binaries, you must downgrade the feature
compatibility version and remove any 4.0 features incompatible with 3.6 or earlier versions as outlined
below. These steps are necessary only if
featureCompatibilityVersion
has ever been set to "4.0"
.
1. Downgrade Feature Compatibility Version
Connect a
mongo
shell to themongos
instance.Downgrade the
featureCompatibilityVersion
to"3.6"
.db.adminCommand({setFeatureCompatibilityVersion: "3.6"}) The
setFeatureCompatibilityVersion
command performs writes to an internal system collection and is idempotent. If for any reason the command does not complete successfully, retry the command on themongos
instance.
To ensure that all members of the sharded cluster reflect the updated
featureCompatibilityVersion
, connect to each shard replica set
member and each config server replica set member and check the
featureCompatibilityVersion
:
Tip
For a sharded cluster that has access control enabled, to run the following command against a shard replica set member, you must connect to the member as a shard local user.
db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )
All members should return a result that includes:
"featureCompatibilityVersion" : { "version" : "3.6" }
If any member returns a featureCompatibilityVersion
that includes
either a version
value of "4.0"
or a targetVersion
field,
wait for the member to reflect version "3.6"
before proceeding.
For more information on the returned featureCompatibilityVersion
value, see Get FeatureCompatibilityVersion.
2. Remove Backwards Incompatible Persisted Features
Remove all persisted features that are incompatible with 4.0. For example, if you have defined any view definitions, document validators, and partial index filters that use 4.0 query features such as the aggregation convert operators, you must remove them.
If you have users with only SCRAM-SHA-256
credentials, you should
create SCRAM-SHA-1
credentials for these users before downgrading.
To update a user who only has SCRAM-SHA-256
credentials, run
db.updateUser()
with mechanisms
set to SCRAM-SHA-1
only and the pwd
set to the password:
db.updateUser( "reportUser256", { mechanisms: [ "SCRAM-SHA-1" ], pwd: <newpwd> } )
Procedure
Downgrade a Sharded Cluster
Warning
Before proceeding with the downgrade procedure, ensure that all
members, including delayed replica set members in the sharded
cluster, reflect the prerequisite changes. That is, check the
featureCompatibilityVersion
and the removal of incompatible
features for each node before downgrading.
Note
If you ran MongoDB 4.0 with authenticationMechanisms
that included SCRAM-SHA-256
, omit SCRAM-SHA-256
when
restarting with the 3.6 binary.
Download the latest 3.6 binaries.
Using either a package manager or a manual download, get the latest release in the 3.6 series. If using a package manager, add a new repository for the 3.6 binaries, then perform the actual downgrade process.
Once upgraded to 4.0, if you need to downgrade, we recommend downgrading to the latest patch release of 3.6.
Disable the Balancer.
Connect a mongo
shell to a mongos
instance in
the sharded cluster, and run sh.stopBalancer()
to
disable the balancer:
sh.stopBalancer()
Note
If a migration is in progress, the system will complete the
in-progress migration before stopping the balancer. You can run
sh.isBalancerRunning()
to check the balancer's current
state.
To verify that the balancer is disabled, run
sh.getBalancerState()
, which returns false if the balancer
is disabled:
sh.getBalancerState()
For more information on disabling the balancer, see Disable the Balancer.
Downgrade each shard, one at a time.
Downgrade the shards one at a time. If the shards are replica sets, for each shard:
Downgrade the secondary members of the replica set one at a time:
Shut down the
mongod
instance and replace the 4.0 binary with the 3.6 binary.Start the 3.6 binary with the
--shardsvr
and--port
command line options. Include any other configuration as appropriate for your deployment, e.g.--bind_ip
.mongod --shardsvr --port <port> --dbpath <path> --bind_ip localhost,<hostname(s)|ip address(es)> Or if using a configuration file, update the file to include
sharding.clusterRole: shardsvr
,net.port
, and any other configuration as appropriate for your deployment, e.g.net.bindIp
, and start:sharding: clusterRole: shardsvr net: port: <port> bindIp: localhost,<hostname(s)|ip address(es)> storage: dbpath: <path> Wait for the member to recover to
SECONDARY
state before downgrading the next secondary member. To check the member's state, you can issuers.status()
in themongo
shell.Repeat for each secondary member.
Step down the replica set primary.
Connect a
mongo
shell to the primary and users.stepDown()
to step down the primary and force an election of a new primary:rs.stepDown() When
rs.status()
shows that the primary has stepped down and another member has assumedPRIMARY
state, downgrade the stepped-down primary:Shut down the stepped-down primary and replace the
mongod
binary with the 3.6 binary.Start the 3.6 binary with the
--shardsvr
and--port
command line options. Include any other configuration as appropriate for your deployment, e.g.--bind_ip
.mongod --shardsvr --port <port> --dbpath <path> --bind_ip localhost,<hostname(s)|ip address(es)> Or if using a configuration file, update the file to include
sharding.clusterRole: shardsvr
,net.port
, and any other configuration as appropriate for your deployment, e.g.net.bindIp
, and start the 3.6 binary:sharding: clusterRole: shardsvr net: port: <port> bindIp: localhost,<hostname(s)|ip address(es)> storage: dbpath: <path>
Downgrade the config servers.
If the config servers are replica sets:
Downgrade the secondary members of the replica set one at a time:
Shut down the secondary
mongod
instance and replace the 4.0 binary with the 3.6 binary.Start the 3.6 binary with both the
--configsvr
and--port
options. Include any other configuration as appropriate for your deployment, e.g.--bind_ip
.mongod --configsvr --port <port> --dbpath <path> --bind_ip localhost,<hostname(s)|ip address(es)> If using a configuration file, update the file to specify
sharding.clusterRole: configsvr
,net.port
, and any other configuration as appropriate for your deployment, e.g.net.bindIp
, and start the 3.6 binary:sharding: clusterRole: configsvr net: port: <port> bindIp: localhost,<hostname(s)|ip address(es)> storage: dbpath: <path> Include any other configuration as appropriate for your deployment.
Wait for the member to recover to
SECONDARY
state before downgrading the next secondary member. To check the member's state, issuers.status()
in themongo
shell.Repeat for each secondary member.
Step down the replica set primary.
Connect a
mongo
shell to the primary and users.stepDown()
to step down the primary and force an election of a new primary:rs.stepDown() When
rs.status()
shows that the primary has stepped down and another member has assumedPRIMARY
state, shut down the stepped-down primary and replace themongod
binary with the 3.6 binary.Start the 3.6 binary with both the
--configsvr
and--port
options. Include any other configuration as appropriate for your deployment, e.g.--bind_ip
.mongod --configsvr --port <port> --dbpath <path> --bind_ip localhost,<hostname(s)|ip address(es)> If using a configuration file, update the file to specify
sharding.clusterRole: configsvr
,net.port
, and any other configuration as appropriate for your deployment, e.g.net.bindIp
, and start the 3.6 binary:sharding: clusterRole: configsvr net: port: <port> bindIp: localhost,<hostname(s)|ip address(es)> storage: dbpath: <path>
Re-enable the balancer.
Once the downgrade of sharded cluster components is complete, re-enable the balancer.
Note
The MongoDB 3.6 deployment can use the BinData resume tokens
returned from a change stream opened against the 4.0 deployment, but
not the v0
or the v1
hex-encoded string resume tokens.