setting up Neo4j replication on two instances - neo4j

I am planning to configure some sort of 2 node replication for neo4j, similar to mysql replication. Since I am a little constrained on resources I don't want to pay for more than two Cloud compute instances. Also I am happy with just one real time or near real time copy of the neo4j database. So the approach i can think of is:
Configure HA on the two compute nodes with the help of an arbiter instance. Setup one neo4j instance (master) on first node and another neo4j instance (slave) + another neo4j instance (arbiter, only for arbitration, no data logging) instance on second node.
OR
Setup a cron for online backup using the neo4j-backup tool. Setup incremental backups every hour or so. Not sure the load it may put on the prod server, planning to test that out.
I am more inclined on the first approach since I get a more real time copy the database (I also get HA/load balancing with instant failover but that is not a priority right now).
Please let me know
which of the two approach is better,
if there is another way to achieve the same or
if any of the above approaches are not suitable or have some flaws.
I am a little new to Neo4j HA so please pardon me for my ignorance. Thanks !

So. You already mentioned available solutions.
TL;DR; I prefer first option.
Cluster
In general, recommended layout is 3 nodes (2 slaves + 1 master).
But your layout - 2 nodes (1 master + 1 slave + 1 arbiter) is viable too. Especially if one server can handle your workload.
Good things:
Almost "real-time" replica.
Possibility to utilise resources to handle bigger workload.
Better availability.
Notes:
If you have 10mb/sec write load on master, then same load will be applied on slave node. This shouldn't affect reads from slave at all (except write load is REALLY huge).
Maintenance costs are bigger, then single-instance installation. You should plan how to handle cluster upgrades, configuration updates, plugin updates.
Branched data. In clustered environment there is possibility to end up in "split-brain" scenario, when 2 nodes have different data and decision should be made which data should be kept. Neo4j handles such cases quite good. But you should keep in mind that small data-loss can occur in VERY RARE scenarios.
Backup
Good things:
Simple. Just do backups from database.
Consistency check. When backup is made, tool runs consistency check to verify if database is not damaged. There is no possibility that Backup will screw up live database. If there any issues - you will be notified via logs from backup utility. See below detailed info on to how backup is performed.
Database. Neo4j backup is fully-functional database. You can spin-up server that points to backup database, and do everything you wan't.
Incremental backups. You can do incremental backups as often, as you wan't.
Notes:
Neo4j scales vertically very well (depends on size of database). It can handle huge load on single instance (we had up to 3k requests/second on medium machine). So, you can get one bigger machine for Neo4j server and other smaller (cheaper) for backups.
How backup is performed?
One thing that should be kept in mind - live database is still fully operational. Backup utility doesn't not stop or prevent any actions.
When transaction in database is committed, all changes are appended to transaction log.
When there are no previous backup present: copy whole storage.
When there is previous backup AND transaction logs are available: copy new transaction logs and replay them on to storage.
When there is previous backup AND transactions are NOT available: discard existing storage, copy existing storage.
Why transaction logs can not be available? Your configuration may say to keep only latest transaction logs (i.e. 1 hour), or not to keep at all.
Relevant settings:
keep_logical_logs
logical_log_rotation_threshold
Other
Anyway, you should consider making backups event in clustered environment. Everything can fail, in any moment.
In general - everything depends on your load and database size.
If your database is small enough to fully fit in memory and one machine is enough to handle all load, then one Neo4j instance will be enough. Just do backup.
If you wan't better scalability/availability and real-time working replica, then cluster setup is best choice.

Related

How to take a snapshot of neo4j database

I see there is a tool that allows for backups to be taken of a running Neo4J database, either via Java or via the backup tool.
The backup will obviously take some time to complete, during which time additional nodes may be added, modified or deleted. Is it possible to take a snapshot of the graph database at a particular instant in time?
My use case: N4J is used to store events, which are stored elsewhere. I'd like to take a snapshot of the graph at an instant in time, then when it's restored at a later date, know what was missing from the graph based on when the backup was taken and be able to reconstruct a complete version of the database that is accurate to the present time by adding the missing events.
There's a related question that has good discussion of this, let me cut to the chase.
If you're using the commercial version of neo4j, then neo4j backup options and/or the backup tool are your best options.
If you're using community edition, then you can't do online backups at present. I have several applications that run using neo4j community, and we have a cron job that runs at 03:00. It shuts down the application, and creates a copy of the database in another location (by copy, I mean it actually creates a tar.gz archive of the DB directory). After this is completed with other maintenance, the application gets restarted again.
Depending on file copy performance and DB size, this isn't too bad. We have a moderate sized DB and we simply accept about 10 minutes of downtime every night.
The neo4j-backup tool is part of Neo4j Enterprise edition. It takes a backup consistently at the time you've started it. After backup is finished a verbose consistency check is run to validate recoverability. It works either as full backup or incrementally.
This tool does not incorporate restoring for a given point in time or comparing with other backups. A point-in-time restore can be achieved by combining it with a classic file backup tool. I've made good experience with backup2l. neo4j-backup would started as part of backup2l's PRE_BACKUP. The same approach should work with any other backup tool out there.
Using your backup tool you can retrieve the full graph.db directory at a desired point-in-time from your archives and use them.

Temporarily stop mnesia replication

I have an erlang application that uses mnesia to store some basic state that defines users and roles of our system. We have a new feature that we need to roll out that requires an extension of the record schema stored in one our mnesia tables.
Our deployment plan was to take one node out of the cluster (just by removing from network), deploy the code, run a script to upgrade the record schema on that node. Bring it back into service. However, once I upgrade the records on this node, it replicates to the other nodes and certain operations begin failing on those nodes because of the mis-matched record schema. Obviously a BIG PROBLEM for zero-down-time deployments.
Is there a way to isolate my schema changes so that the schema upgrade can be run on each node as they are upgraded? Preferably for only the table being upgraded, allowing the other tables to keep replicating. However, I could live with shutting-of replication between all nodes for the few minutes it takes for use to deploy to all nodes.
I had this exact problem. The only way I was able to solve it was to take all nodes out of the cluster and leave only one live, upgrade that "master" node's schema and code, which can hopefully be done while live, then for each remaining node, delete its database files, upgrade the code, and bring the node up (creating the tables with the correct new schema) and back into the cluster.
I used an escript I wrote that adds and removes nodes from a cluster to make this easier, and an Ansible playbook to orchestrate it. I really don't want to do that again any time soon.
The essential problem is that Mnesia doesn't have schema versioning, otherwise this could be done in a much better way.

One replicated mnesia table has become out-of-sync

I have an erlang application currently running on four nodes with a replicated mnesia db that stores minimal data regarding connected clients. The mnesia replication has been working seamlessly in the past (as far as I know anyway) but a client recently noticed that one of the nodes is missing some ids related to his application.
I'm not really sure how this happened. Our network may have had a hiccup at the time. Maybe? But, of more urgency at the moment is getting the data into a good state across all nodes. Is there a way to tell mnesia to replicate from a known-good node?
Mnesia is legendary about this issue. It's a huge PITA.
Looking at it from CAP theorem's point of view, most systems built with Mnesia end up being C-A (consistency-availability with no partition tolerance) systems. For most of the time you have (and heavily rely on) its hard consistency. Then a network partition happens...
It's still available for writes, but these writes destroy consistency. And later on, Mnesia has no mechanism for automatic data repair.
Everyone who uses Mnesia in a cluster should familiarize themselves with these tradeoffs. Your problem is a clear sign that using Mnesia was a poor choice. Double so if this data is critical to you.
I too use Mnesia in such a way (sometimes we all need speed you know). But I make sure to only use it to store data that I can easily reconstruct. In general, if you need it stored on disk, Mnesia is no good, except for toy projects.
I make sure to always have this function at hand:
reinit_mnesia_cluster() ->
rpc:multicall(mnesia, stop, []),
AllNodes = [node() | nodes()],
mnesia:delete_schema(AllNodes),
mnesia:create_schema(AllNodes),
rpc:multicall(mnesia, start, []).
Use it only after the network partition has been resolved and all nodes are reachable. This will erase all Mnesia replicas and start it anew. Again, if you can't live with what it does, then using Mnesia was a poor choice.
For important data that needs hard consistency, use SQL. For important data that needs availability, use Riak. For shared state that needs speed, use Redis. Mnesia is no replacement for these systems, although at first it does seem so.
Edit on 2014-11-16: Here is a much better article on the topic, explaining in detail what I said above https://medium.com/#jlouis666/mnesia-and-cap-d2673a92850
Honestly, I think the cleanest way to get an out-of-sync Mnesia to replicate from a known good node is to shut down the application on the bad node, and delete all its Mnesia database files, then do the following.
Write an escript that starts Mnesia up standalone using the "bad" node name and Mnesia directory, replicates the tables from a known good node, and shuts Mnesia down. Run that escript on the bad node.
The act of replicating the tables and shutting Mnesia down gracefully puts the node back in sync with the cluster. Then, when you start the application up on the bad node, it will join up and stay in sync with the cluster.
Of course, this description lacks precise details, but that's the gist of it. There are surely less brute force ways of doing this, but unless you have massive amounts of data to replicate, I think this way is the quickest and cleanest.

Postgresql replication in rails with data-fabric gem

I am currently setting up a master-slave app using Ruby on Rails. I am planning to use data-fabric or octopus gem for handling the read/write connections.
This is my first time setting up master-slave DBs. I am confused over the various open source tools available to implement the postgresql replication e.g. pgpool II, pgcluster, Bucardo and Hot Standby/Streaming Replication (built in feature in postgresql 9.1)
My requirements are
fault tolerance(high availability and no data loss on failover)
load balancing
Thanks in advance
Note: I have gone through the stackoverflow post regarding postgresql replication but they are pretty old and not helping to conclude on which tool I should go with.
In your case, streaming replication is the place to start. It is not very flexible but it does what you need regarding database reads as long as you don't need to replicate between major versions.
Database Replication 101
Database replication is a way to ensure that data saved to a specific server becomes stored in a number of other servers. This is often done to better utilize more limited network connections, ensure fault tolerance (so there is essentially a hot back-up), ensure that read-only queries can be distributed over a larger number of databases, etc. This all must be done without sacrificing the the basic guarantees of ACID.
There are a number of different overlapping ways to categorize replication solutions. These include:
Page or file-level vs row-level vs statement-level
Synchronous vs Asynchronous
Master-slave vs Multi-Master
In general understanding replication and the tradeoffs between solutions requires relatively strong understanding of database mechanics and ACID guarantees. I will assume you are relatively familiar with storage mechanics, and deterministic vs non-deterministic operations and the like.
What is Being Replicated? File changes (Physical) vs Row Changes (Logical) vs Statements
The simplest approach is to replicate block changes to files, for example as stored in the write-ahead log in PostgreSQL. This replicates changes at the page level and it requires identical file formats. This means you cannot replicate across major versions, CPU architectures, or operating systems. Anything that could affect the alignment of tuples, for example, will cause the replication to either fail or, worse, corrupt the slave's database. This is the approach streaming replication uses. It is simple to set up, and it always replicates everything in the database cluster.
Additionally this approach means you can easily guarantee that the master and slave databases are identical down to the file level. Because of the fact that the PostgreSQL WAL is cluster-global it is unlikely that this approach will ever replicate anything short of the entire database cluster.
As a description of how this works, suppose I:
UPDATE my_table SET rand_value = random() WHERE id > 10000;
In this case, this changes a bunch of data pages and the file operations are replicated to the replicas. The files remain identical between the master and slave.
Another approach, one taken by Slony, Bucardo, and others is to replicate rows in a logical manner. In this approach, changed rows are flagged and logged, and the changes sent to the replicas. The replicas re-run row operations from the master database. Because these are add-on tools which do not replicate file operations but rather logical database operations, they can replicate across CPU architectures, operating systems, etc. Also they are usually designed so that you can replicate some but not all tables in a database, allowing for a lot of flexibility. On the other hand this leads to a lot of potential for errors. "Oops, that table was not replicated" is a real problem.
In this case when I run the update statement above, a trigger is fired capturing the actual rows inserted and deleted and these are logged, replicated, and the row operations re-run. Because this happens after rand() is run, the databases are logically, but not necessarily physically identical.
A final approach is statement replication. In this case we replicate statements and re-run the statements on the replicas. Some configurations of PgPool will do this. In this case, you cannot ensure that a database is logically equivalent to its replica if any non-deterministic functions are run. In the statement above, the statement itself will run on each replica, ensuring different pseudorandom numbers in the relevant column.
Synchronous vs Asynchronous
This distinction is important to understand regarding failover guarantees. In an asynchronous replication system, the updates are queued and transferred when possible to the replicas and re-run there. In a synchronous replication system the database which accepts the write will not return a successful commit until at least a certain number of replica databases report a successful commit.
Asynchronous replication is generally more robust and produces better availability than synchronous replication. This is because synchronous replication introduces additional points of failure. If you have one master and one slave, then if either system goes down, your database becomes unavailable at least for write operations.
The tradeoff though is that synchronous replication offers a guarantee that data which is committed is in fact available on replicas in the event that the master, say, suffers catastrophic hardware failure immediately following commit. This is a very low probability event, but in some cases it is important that you know the data is still available. In short this provides additional durability guarantees not present in async replication.
Multi-Master vs Master-Slave
Most replication systems are master-slave. In this case, all writes begin at one node and are replicated to other nodes. Writes may only begin at one node. They may not begin at other nodes. This makes replication straight-forward because we know that the slaves represent a past state of the master.
Multi-master replication allows writes to occur to more than one node. In an asynchronous replication system, this leads to the problem of conflict resolution. These problems are actually worse than most assume when you add DDL statements. Suppose two different users run the above update statement on two different masters. We will now have a set of records that have to be replicated across but they will conflict.
Multi-master replication typically requires that people think through this conflict resolution process quite carefully. It is never a process that just works out of the box. Often times you write your own conflict resolution routines. For this reason I typically recommend avoiding multi-master replication unless you really need it.

How to prepare for data loss in a production website?

I am building an app that is fast moving into production and I am concerned about the possibility that due to hacking, some silly personal error (like running rake db:schema:load or rake db:rollback) or other circumstance we may suffer data loss in one database table or even across the system.
While I don't find it likely that the above will happen, I would be remiss in not being prepared in case it ever does.
I am using Heroku's PG Backups (which is to be replaced with something else this month), and I also run automated daily backups to S3: http://trevorturk.com/2010/04/14/automated-heroku-backups/, successfully generating .dump files.
What is the correct way to deal with data loss on a production app?
How would I restore the .dump file in case I need to? Can I do a selective restore if a small part of the system is hit?
In case a selective restore is not possible: assume one table loses data 4 hours after the last backup. Result => would fixing the lost table require rolling back 4 hours of users' activity? Any good solution to this?
What is the best way to support users through the inconvenience if something like this happens?
A full DR (disaster recovery) solution requires the following:
Multisite. If a fire, flood, Osama Bin Laden or whathaveyou strikes the Amazon (or is it Salesforce?) data center that Heroku uses, you want to be sure that your data is safe elsewhere.
On-going replication of the data to a separate site (or sites). That means that every transaction that's written to your database on one site, is replicated within seconds to the mirror database on the other site. Most RDBMS's have mechanisms to let you do a master-slave replication like that.
The same goes for anything you put on a filesystem outside of the database, such as images, XML configuration files etc. S3 is a good solution here - they replicate everything to multiple data centers for you.
I won't hurt to create periodic (daily or so) dumps of the database and store them separately (e.g. on S3). This helps you recover from data corruption that propagates to the slave DBs.
Automate the process of data recovery. You want this to just work when you need it.
Test everything. Ideally, you want to automate the test process and run it periodically to ensure that your backups can restore. Netflix Chaos Monkey is an extreme example of this.
I'm not sure how you'd implement all this on Heroku. A complete solution is still priced out of reach for most companies - we're running this across our own data centers (one in the US, one in EU) and it costs many millions. Work according to the 80-20 rule - on-going backup to a separate site, plus a well tested recovery plan (continuously test your ability to recover from backups) covers 80% of what you need.
As for supporting users, the best solution is simply to communicate timely and truthfully when trouble happens and make sure you don't lose any data. If your users are paying for your service (i.e. you're not ad-supported), then you should probably have an SLA in place.
About backups, you cannot be sure at 100 percent every time that no data will be lost. The best is to test it on another server. You must have at leat two types of backup :
A database backup, like pg-dump. A dump is uniquely SQL commands so you can use it to recreate the whole database, just a table, or just a few rows. You loose the data added in the meantime.
A code backup, for example a git repository.
in addition to Hartator's answer:
use replication if your DB offers it, e.g. at least master/slave replication with one slave
do database backups on a slave DB server and store them externally (e.g. scp or rsync them out of your server)
use a good version control system for your source code, e.g. Git
use a solid deploy mechanism, such as Capistrano and write your custom tasks, so nobody needs to do DB migrations by hand
have somebody you trust check your firewall setup and the security of your system in general
The DB-Dumps contain SQL-commands to recreate all tables and all data... if you were to restore only one table, you could extract that portion from a copy of the dump file and (very carefully) edit it and then restore with the modified dump file (for one table).
Always restore first to an independent machine and check if the data looks right. e.g. you could use one Slave server, take if offline, then restore there locally and check the data. Good if you have two slaves in your system, then the remaining system has still one master and one slave while you restore to the second slave.
To simulate a fairly simple "total disaster recovery" on Heroku, create another Heroku project and replicate your production application completely (except use a different custom domain name).
You can add multiple remote git targets to a single git repository so you can use your current production code base. You can push your database backups to the replicated project, and then you should be good to go.
The only step missing from this exercise verses a real disaster recovery is assigning your production domain to the replicated Heroku project.
If you can afford to run two copies of your application in parallel, you could automate this exercise and have it replicate itself on a regular basis (e.g. hourly, daily) based on your data loss tolerance.

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