Coda is a network file system based on the Andrew File System (AFS) that was designed to be highly secure, available, and transparent to users. It overcomes limitations of AFS by replicating file volumes across multiple servers for high availability and caching files at client machines to enable disconnected operation. Coda uses remote procedure calls and file caching at clients to provide transparent access to files even when disconnected from the server network, making it suitable for mobile and disconnected use.

1. Presented By:Presented By:
Samreen TahirSamreen Tahir

2. Coda is a network file system and a descendent of the Andrew File SystemCoda is a network file system and a descendent of the Andrew File System
2. It was designed to be:2. It was designed to be:
 Highly secureHighly secure
 AvailableAvailable
 Transparent to its users.Transparent to its users.
WHAT IS IT?WHAT IS IT?
Andrew File System:Andrew File System:
• Architecture of Coda is based on Andrew File SystemArchitecture of Coda is based on Andrew File System
• AFS was developed at Carnegie Mellon UniversityAFS was developed at Carnegie Mellon University
• It was designed to support the CMU community that meant serving 10, 000 workstationsIt was designed to support the CMU community that meant serving 10, 000 workstations

3. Limitations of AFS:Limitations of AFS:
 Limited form of replication posed scalingLimited form of replication posed scaling
problemsproblems
 Non-availability of services when serversNon-availability of services when servers
and network components failand network components fail
 No catering for mobile use of portableNo catering for mobile use of portable
computerscomputers
Coda was designed to overcome the problems associatedCoda was designed to overcome the problems associated
with AFS.with AFS.

4. It works by implementing two COMPLEMENTARYIt works by implementing two COMPLEMENTARY
functionalities :functionalities :
1.1. Availability of files by replicating a file volume acrossAvailability of files by replicating a file volume across
many serversmany servers
2. Disconnected mode of operation by caching files at the2. Disconnected mode of operation by caching files at the
client machineclient machine
How does Coda work?How does Coda work?

5. The Coda architecture is based on AFS: The Andrew File SystemThe Coda architecture is based on AFS: The Andrew File System
Transparent access
to a Vice file server
Vice file serverVirtue Client
1. Vice nodes: dedicated filededicated file
serversservers
AFS is divided into two types
of nodes:
2. Virtue nodes: client machinesclient machines

6. User
process
Venus
process
User
process
RPC
client stub
Virtual file
system layer
Network
Virtue Client Machine
The internal organization of a virtue workstation:The internal organization of a virtue workstation:
Local OS
Local file
system
interface
Venus: A process that is hosted on each Virtue system.
VFS: Virtual File System, intercepts the calls from client application or user processes
and forwards them either to the local file system or Venus.

7. How do the server and client communicate in Coda?How do the server and client communicate in Coda?
RPC call
Client
File Server
(Blocking)
Reply May take arbitrary
amount of time
Answer: Through Remote Procedure Calls (RPC)Answer: Through Remote Procedure Calls (RPC)
1.1. The server keeps sending back messages to the client that it is still workingThe server keeps sending back messages to the client that it is still working
on the problem.on the problem.
2. If the server dies and the client notices that it is not receiving any messages2. If the server dies and the client notices that it is not receiving any messages
it reports back failure to the client application.it reports back failure to the client application.

8. (contd.)(contd.)
File server
client
Invalidate F
Invalidate F
Invalidate FF
F
F
F
F
F
File
Updated File F
When a server notices updates in a file, it must inform the client whichWhen a server notices updates in a file, it must inform the client which
are caching a copy of it to invalidate that copy.are caching a copy of it to invalidate that copy.

9. (contd.)(contd.)
There are two ways to invalidate a file:
Sending invalidation message one
at a time.
Sending invalidation messages in
parallel.
Time
Client
Client
Server
invalidate
reply
reply
invalidate
This method is inefficient since a client may crash before
giving a reply to the server.
A better approach:
COMMUNICATION:COMMUNICATION:
Time
Client
Client
Server
invalidate
reply
reply
invalidate

10.  Coda uses RPC2: more sophisticated than theCoda uses RPC2: more sophisticated than the
traditional RPC used by NFStraditional RPC used by NFS
 RPC2 implements a functionality calledRPC2 implements a functionality called sideside
effecteffect: a separate connection is opened when a: a separate connection is opened when a
remote procedure is called.remote procedure is called.
 This separate connection is for the applicationThis separate connection is for the application
specific protocol between the client and thespecific protocol between the client and the
server.server.

11. RPC2RPC2 contd.
Server side
effect
RPC server
stub
RPC client
stub
Client side
effect
Application-specific protocol
Side-effects in Coda’s RPC2 system.
RPC
RPC protocol
Client Application
Server

12. Naming:Naming:
 Coda uses name space implementationCoda uses name space implementation
analogous to that of UNIX.analogous to that of UNIX.
 Volumes are the building units of theVolumes are the building units of the
entire file system and are mounted at aentire file system and are mounted at a
mount point.mount point.

13. Clients in coda have access to aClients in coda have access to a
single shared namespacesingle shared namespace
Client A Client BServer
Network
Exported directory
mounted by client
Exported directory
mounted by client
Naming inherited from server’s namespace
Clients in Coda have access to a single shared namespace.
afs local
bin
bin
afspkg
pkg

14. File Identifiers:File Identifiers:

15. Client Caching:Client Caching:

16. Server Replication:Server Replication:

17. Fault Tolerance:Fault Tolerance:
Disconnected OperationDisconnected Operation

18. Fault Tolerance:Fault Tolerance:
Disconnected OperationDisconnected Operation