ITIS 3110 Jason Watson

Replication methods o Primary/Backup o Master/Slave o Multi-master Load-balancing methods o DNS Round-Robin o Reverse Proxy

Duplication of data between multiple machines Two main uses o Failure mitigation o Load balancing

Primary/Backup Master/Slave Multi-Master

Ensures availability of a ‘resource’ Only one server provides access to resource at a time Does not provide any load balancing Uses o Services which do not understand replication o Services which would be too expensive to replicate  Too much active state

Primary server is active o Has exclusive lock on resource Backup server is on standby Heartbeat between Primary and Backup o Allows each server to keep track of other’s state o Heartbeat can be in-band or out-of-band  In-band: e.g. over same network link  Out-of-band: e.g. serial, secondary network connection, etc.

When heartbeat lost: Backup steals lock on resource o Ensures primary is down (e.g. cut power) o Performs recovery on resource (e.g. fsck) o Backup may even steal primary’s MAC address! Backup becomes primary Old primary will become backup once heartbeat re-established

Client or Router PrimarySecondary Resource Heartbeat

File Servers o Disk array is the resource Virtual Machine Servers o Virtual Machine is the resource

All servers can be read Only master server accepts writes Can provide rudimentary load balancing Must be implemented in a protocol Uses o Stateless, Read-Heavy protocols

Can be push or pull o Master pushes changes to slaves o Slaves pull changes from master

Most protocols support incremental updates o Every change is versioned o Master keeps a journal of changes o Slave asks for updates since a particular version o Master replays journal from version requested All protocols support full updates o Master sends complete data set to slave

MasterSlave Read Write Updates

DNS Kerberos LDAP Databases

Any node can accept read or writes Will distribute writes to all other nodes Very complicated to implement Cluster file systems use this method

Multi-master replication technique Shares memory space between multiple machines Common usage is to distribute HTTP session state between web servers

Load balancing distributes load across multiple servers Often relies on replication to distribute data to the multiple servers

DNS Round-Robin Reverse Proxy

Simplest Method Multiple DNS records for a single address DNS server will return addresses in different order every time it is requested Does not assign work based on load Can not detect failed servers

DNS Kerberos LDAP SMTP HTTP

SMTP does not require replication Servers receive and transmit mail They do not need to all have the same mail messages

HTTP is an odd beast Purest form is stateless o Shared file system is sufficient Sessions, other state complicates matters o Reverse Proxy is simpler o DNS Round-Robin + Distributed Shared Memory works too

Device that sits in front of a group of servers and direct traffic o Client traffic is terminated on load balancer o Load balancer opens new connection to one back-end server and forwards client data Often used for Web servers Many companies provide ‘turn-key’ solutions Can make intelligent decisions when forwarding traffic to servers o Send less traffic to heavily loaded servers o Skip dead servers

SSL endpoint Compression Caching Intrusion Prevention

Client Server Server Server Reverse Proxy

FTP HTTP

Many HTTP reverse proxies support session pinning Session pinning o A user session will always be forwarded to same server o Allows that server to keep track of session data for that user