mongoseal

mongoseal

Distributed locks using mongodb, with fencing

Setup

Installing:

go get -u github.com/aarondwi/mongoseal

Run this script below in your mongodb

db.lock.createIndex( { "Key": 1 }, { unique: true } )
db.lock.createIndex( { "last_seen": 1 }, { expireAfterSeconds: 600 } )

The 1st one is required, to ensure key uniqueness

The 2nd one is used to remove old entry that are not deleted (maybe because of latency, process died, etc). The expireAfterSeconds should be set to duration considered safe if the lock get acquired by the 2nd or so process.

There is an optional parameter, needRefresh, which can auto refresh the lock just before it expired. The refresh starts at expireAfterSeconds - remainingBeforeResfreshSecond, and then loop for every expireAfterSeconds

Notes

Even though this distributed lock implementation use fencing, but fencing without application specific semantic may still fail to provide exclusivity (see comments here). The goal of 2 types of timeouts (database level expire and application level timeout) are different:

1. the database level expire to ensure the storage requirement does not grow unbounded. Consider setting this value to a number considered safe if 2 workers hold the locks
2. the application level timeout mainly used for generating fencing token. Here, multiple workers can still hold the locks, but you have fencing token (from lock.Version) to be used for checking at storage/database level.

The application's OS time should be synced with NTP, and as long as the application's time is ntp-correct, this implementation should also is

Write operations are using majority concern, while read operations are using linearizable. Because of this, ensure that your mongo is a replica set, not a standalone.

Usage

// lockExpiryTimeSecond should be set to be far more
// than required duration of a process
//
// For example, if your code gonna need 10s for processing
// and 1s to save it to db or others
// set the expiryTimeSecond to be more than 11s, preferably around 20s
// to add buffer for process pause, network delay, etc
m, err := New(
  connUrl,
  dbname,
  workerUniqueId,
  lockExpiryTimeSecond,
  needRefresh,
  remainingBeforeResfreshSecond)
if err != nil {
  // handle the errors, failed creating connection to mongodb
}

mgolock, err := m.AcquireLock("some-key")
if err != nil {
  // failed acquiring lock, maybe some others have taken it
  // or there is some error in-between
}

if mgolock.IsValid() {
  // your code goes here
  // always need to check IsValid() before starting
  // to ensure the lock doesn't expire even before the code starts
  // can't do anything if the lock becomes invalid in the middle of your code
  // see https://martin.kleppmann.com/2016/02/08/how-to-do-distributed-locking.html

  // dont forget to use fencingToken to make your application safer
  fencingToken := mgolock.Version
}

// release the lock
// it returns nothing, as error may mean some other workers has taken the lock already
m.DeleteLock(mgolock)

See main_test.go for examples

Queries use internally

acquire_lock:

db.lock.update({
  key: 'random-id',
  $or: [
    {last_seen: null},
    {last_seen: {$lt: new Date() - expiryTimeSecond}}]
}, {
  $inc: {version: 1},
  $set:{'owner': 'me', last_seen: new Date()}},
{upsert: true})

get_lock_data:

db.lock.find({key: 'random-id', 'owner': 'me'}, {_id: 0})

delete_lock:

db.lock.remove({key: 'random-id', 'owner': 'me', version: 1})

refresh_lock:

db.lock.update(
  {key: 'random-id', 'owner': 'me', version: 1},
  {$inc: {last_seen: expiryTimeSecond}}
)

Document Schema

{
  "version": 1,
  "owner": "random id generated by each lock, or supplied",
  "key": "unique id for your resource",
  "last_seen": "to check for expiry (probably stale, but other workers may still assume they have it)"
}