The Fluid

Architecture

Your Host Tonight

Image source: my 6 yo daughter

Alex Fernández

Developer with 15+ years of experience

@pinchito

What We will cover

How to Flow

Flow with Requirements

Flow with Operational Constraints

Some Relevant Case Studies

Migration Strategies

Don't Stop Flowing

How to Flow

Turbulent Flow is Irreversible

It can be fun, but we don't want that kind of fun

Laminar Flow is Reversible

And we like when things flow smoothly!

Migrations are hard?

Thermo to the rescue!

Reversible processes are optimal:

no turbulence,
minimal entropy,
less complexity,
reduced headaches!

Change without pain

Go from A to B in a reversible way

(mostly)

Find your cruise velocity

Prepare a reversal strategy

Flow with Requirements

Circumstances Change

And you should adapt to them

Fashion in ARchitecture

80s: minicomputers & terminals

90s: client-server

00s: three tiers

10s: NoSQL

The Perfect Architecture

Does not exist

Modern Architecture

Or so we thought

Slightly More modern architecture

Spot the seven differences

Flow With Constraints

Flow with capacity planning

From 4 to 150+ krps in 2.5 years

Flow for Operational Stability

From 38 to 112 krps in one day

Flow to Lower Costs

How Fast Can You Migrate…

… to a new cloud provider?

… to a new hosting company?

… to your own datacenter?

It matters because costs escalate quickly

Database migrations

are painful efforts

but shouldn't be

How to Migrate Your Database

Build a compatibility layer

Avoid downtime if at all possible

Treat access and data separately

Have a reverse migration strategy…

… but try not to use it

Compatibility Layer

Adapter pattern (remember those?)

Reduced feature set

Don't use new features

Fake missing features

Adapter

Redis to Memcached driver:

exports.RedisAdapter = function(name, address) {
    // self-reference
    var self = this;
        
    // attributes
    var client = driver.getClient(address);
        
    self.get = function(key, callback) {
        runCommand('get', key, function(error, result) {
            if (error) return callback('Could not get ' + key + ':' + error);
            return callback(null, parse(key, result));
    });

    self.set = function(key, value, expiration, callback) {
        if (expiration) {
            return runCommand('set', key, JSON.stringify(value), 'EX', getExpiration(expiration), callback);
        }
        return runCommand('set', key, JSON.stringify(value), callback);
    });
};

Migrate and migrate again

We have used the following databases:

Couchbase
Memcached
Redis
DynamoDB
PostgreSQL
RedShift

Different systems have different trade-offs

and show different failure modes

Case Studies

Warning: may not apply to your situation

MediaSmart Mobile

Migration to Amazon virtual private cloud

Tried on 2015-03-03

Reversed on 2015-03-05

Due to an unrelated failure (!)

Tried again on 2015-03-11

Migrated EU on Friday the 13th

because who's afraid of superstitions?

Instagram

Migrated from AWS to Facebook datacenters

Year-long effort, from 2013-03 to ~2014-03

Had to go through AWS VPC first

Neti — a dynamic iptables manipulation daemon in Python

Three weeks into VPC, two weeks to FB

Bare minimum approach (!)

Big Adtech Company X

Also migrating datacenters

Will start sending 10% of traffic

from new datacenter

No published materials

Inter-datacenter latencies

Migration Strategies

Server: Stop and Migrate

Stop the system
Make a cold copy
Point clients to new database
Start again

Server: Stop and Migrate

settings.js:

module.exports = {
    reidsAddress: 'redis.mydomain.com',
};

db.js:

var settings = require('./settings.js');
exports.db = {
    current: new RedisAdapter(settings.redisAddress);
};

user:

var db = require('./db.js');
db.current.get(key, function(error, result) {
    ...
});

Server: Stop and Migrate

Most basic migration

Requires downtime

Reversal:

Just point your settings to the old address
Stop again and migrate back

Not really reversible

Server: Read-only Version

Switch to read-only
Make a hot copy
Change to new database
Switch back to read and write

Server: Read-only Version

Read-only is not always admissible

A hot copy takes longer than a cold one

Reverse migration: switch to read-only again, migrate

Server: synchronize

Make a hot copy
Synchronize all writes
Switch to new copy when ready

Server: synchronize

Depends on server mechanism

No downtime: cool!

Reversal strategy: synchronize back

Full synchronization is hard!

Server: Double Copy

Make a hot copy
Switch to new database
Make another hot copy

Server: Double Copy

Some data loss is admissible

e.g.: 500M profiles

A timestamp is very valuable

Some data loss is inevitable

Reversal: prepare a reverse copy

Decorator

Just a clever adapter:

var Memcached = require('memcached');

exports.CleverAdapter = function(name, address) {
    // self-reference
    var self = this;
    
    // attributes
    var oldAdapter = new Memcached(address + ':11211');
    var newAdapter = new RedisAdapter(address);
        
    self.get = function(key, callback) {
        if (badWeather()) {
            return oldAdapter.get(key, callback);
        }
        return newAdapter.get(key, callback);
    }
};

Downside: a few µs more per query

Client: Dual Lookup

Read from one database
If not present, try to read from second database

Client: Dual Lookup

db.js:

exports.db = {
    v1: new RedisAdapter(settings.oldRedis),
    v2: new RedisAdapter(settings.newRedis),
};

client:

function get(key, callback) {
    db.v1.get(key, function(error, result) {
        if (error || result) callback(error, result);
        db.v2.get(key, callback);
    });
}

Client: Dual Lookup

Migrate your servers at your leisure

Reversible by design

Now you're talking!

Bad latency issues

from old + new databases

Client: Dual Write

Similar to dual lookup

Latency may not be important

Client: Timed Rollover

If date >

Client: Timed Rollover

client:

var CUTOFF_DATE = '2015-05-13';

function get(key, callback) {
    if (key.substringFrom('#') > CUTOFF_DATE) {
        db.v1.get(key, function(error, result) {
    } else {
        db.v2.get(key, callback);
    });
}

function set(key, value, expiration, callback) {
    if (new Date.toISOString() > CUTOFF_DATE) {
        db.v1.set(key, value, expiration, function(error, result) {
    } else {
        db.v2.set(key, value, expiration, callback);
    });
}