Fine-Grained Failover Using Connection Migration - PowerPoint PPT Presentation

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Fine-Grained Failover Using Connection Migration

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Transparent, possibly mid-stream failover. Requires co ... Stream Mapping. Infer application state from transport layer information. Connection Migration ... – PowerPoint PPT presentation

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Title: Fine-Grained Failover Using Connection Migration


1
Fine-Grained FailoverUsing Connection Migration
  • Alex C. Snoeren,
  • David G. Andersen, Hari Balakrishnan
  • MIT Laboratory for Computer Science

2
The Problem
Client
Content server
More often than users want to know
3
Solution Server Redundancy
Use a healthy one at all times.
4
Failover Components
  1. Health Monitoring
  1. Connection Resumption
  1. Server Selection

5
Todays Replication Technology
  • DNS/Content Routing
  • Wide-area replication
  • Need client awareness
  • Layer 4/Web Switches
  • Transparent, possibly mid-stream failover
  • Requires co-location

Web Switch
6
Ideal Technology
  • Wide area replication
  • Yet somehow synchronize replica servers
  • Transparent failover
  • Enable other servers to continue connections

7
Migrate Architecture
  • Stream Mapping
  • Infer application state from transport layer
    information
  • Connection Migration
  • Transparently hand off sessions between servers

Stream Mapper
Stream Mapper
Stream Mapper
8
Stream Mapping
Client
Server Response
TCP ISS 083521
HTTP 1.1 200 OK Content-Length 328987
... Content-Type video/mpeg
TCP SeqNo 083346
Stream Map
Client Object (URL) Offset (TCP SeqNo)
128.89.3.244234 /StreamingContent.mpg 083346
9
Anatomy of Failover
Client
Support Group
10
Support Groups
  • Set of partially mirrored servers
  • All servers able to provide same content
  • Can be topologically diverse
  • Synchronize on per-connection basis
  • Servers need not be complete mirrors
  • Connections from a failed server can be handled
    by a different support server
  • Connections may have distinct support groups

11
Soft State Synchronization
  • Synchronize within support groups
  • Periodic advertisements
  • Advertise client application object requests
  • Communicate initial transport layer state
  • Only initial state need be communicated
  • Current info inferred from transport layer
  • Clients will reject redundant migrates from stale
    support servers

12
TCP ConnectionMigration
client
server
1. Initial SYN 2. SYN/ACK 3. ACK (with
data) 4. Normal data transfer 5. Migrate
SYN 6. Migrate SYN/ACK 7. ACK (with data)
13
TCP ConnectionMigration
client
server
1. Initial SYN 2. SYN/ACK 3. ACK (with
data) 4. Normal data transfer 5. Migrate
SYN 6. Migrate SYN/ACK 7. ACK (with data)
14
TCP ConnectionMigration
client
server
1. Initial SYN 2. SYN/ACK 3. ACK (with
data) 4. Normal data transfer 5. Migrate
SYN 6. Migrate SYN/ACK 7. ACK (with data)
15
Implementation
  • Software Wedge
  • Stream Mapping
  • Synchronization

Wedge
Server App
Stream Mapping Wedges
Wedge
Server App
Client
16
Wedge Overhead
1e07
Wedge
Direct
1e06
Microseconds per request
100000
10000
1000
1
10
100
1000
10000
Request size (Kbytes)
17
Experimental Topology
Client initiates a transfer to A
Linux/Apache 1.3
128Kbs links
then migrates to B
and back to A
Linux/Apache 1.3
18
Varying Oscillation Rates
1e06
900000
800000
700000
600000
Goodput (bytes)
500000
400000
No Oscillations
10 sec
300000
12 sec
2 sec
5 sec
200000
100000
0
0
10
20
30
40
50
60
Time (secs)
19
Benefits Limitations
  • Enable wide area server replication
  • Low server synchronization overhead
  • Infer current state from transport layer
  • Robust even under adverse loads
  • Health monitors can be overly reactive
  • Gracefully handle cascaded failures
  • Leverages connection migration
  • Requires modern transport stack

20
Networks and Mobile Systems
  • Software available on the web
  • http//nms.lcs.mit.edu/software/migrate
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