Failure Handling in a modal Language

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Failure Handling in a modal Language

Nels Eric BeckmanResearch Talk

Institute for Software ResearchOctober 30, 2006

Failure Handling in a Modal Language

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Claims Made in this Talk

• ML5 is an elegant language for programming distributed systems.

• In the face of node failure, the meaning of ML5 programs becomes unclear.

• We propose extensions to ML5 that makes their meaning clear.• (In reality, this research is a work in

progress.)


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ML5

• A Programming Language for Distributed Systems

• Based on a Modal Logic• i.e. A Logic With an Embedded Notion

of Place

• Tom Murphy’s Thesis Work• Targeted for Grid Programming


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ML5, Briefly...

• Allows Hosts to Send ‘Thunks’ to One Another for Execution• In practice, code can be more cleanly

decomposed.

• Has An Advanced Type System • Location-specific resources can be

typed as so.


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RPC-Style Distributed Programming

PC

Host

Active thread

Blocked thread

Message

fun a = fun b =

rpc(“b”,19.x.x.x) + r

return x;


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PC

Host

Active thread

Blocked thread

Message

fun a = fun b =

rpc(“b”,19.x.x.x) + r

return x;


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PC

Host

Active thread

Blocked thread

Message

fun a = fun b =

rpc(“b”,19.x.x.x) + r

return x;


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PC

Host

Active thread

Blocked thread

Message

fun a = fun b =

rpc(“b”,19.x.x.x) + r

return x;

rpc “b”


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PC

PC

Host

Active thread

Blocked thread

Message

fun a = fun b =

rpc(“b”,19.x.x.x) + r

return x;


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PCPC

Host

Active thread

Blocked thread

Message

fun a = fun b =

rpc(“b”,19.x.x.x) + r

return x;


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PC

PC

Host

Active thread

Blocked thread

Message

fun a = fun b =

rpc(“b”,19.x.x.x) + r

return x;


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PC

Host

Active thread

Blocked thread

Message

fun a = fun b =

rpc(“b”,19.x.x.x) + r

return x;

ret x


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PC

Host

Active thread

Blocked thread

Message

fun a = fun b =

rpc(“b”,19.x.x.x) + r

return x;

ret x


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PC

Host

Active thread

Blocked thread

Message

fun a = fun b =

rpc(“b”,19.x.x.x) + r

return x;

ret x


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ML5 Illustration

PC

Host

Location of thread

Migration of thread


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ML5 Illustration

PC

Host

Location of thread

Migration of thread


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ML5 Illustration

PC

Host

Location of thread

Migration of thread


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ML5 Illustration

PC

Host

Location of thread

Migration of thread


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ML5 Illustration

PC

Host

Location of thread

Migration of thread


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ML5 Illustration

PC

Host

Location of thread

Migration of thread


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ML5 Illustration

PC

Host

Location of thread

Migration of thread


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ML5 Illustration

PC

Host

Location of thread

Migration of thread


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Example

• Remotely Finding List’s Sum (RPC)Server Code:

class ListServ {

List<Integer> myList = new ...

List<Integer> getList() {

return myList; }

}


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Example

• Remotely Finding List’s Sum (RPC)Client Code:class ListClient {

ListServerStub myServ = new ...

public void foo() {

List<Integer> list = myServ.getList();

for(Integer item: list) {

count+= item.intValue();

}

if( count >= 40 )

...

}}


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Example

• Remotely Finding List’s Sum (RPC)• To Fix Should We:

• Add a new server operation that returns true if a list’s sum is greater than 40?• Weird if operation is only used once.• We wouldn’t structure application this

way in a centralized setting.• Bite the performance bullet and send

the whole list?


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Example

• Remotely Finding List’s Sum (ML5)Before:fun foo remote_host remote_list_ref =

let fun sum a_list =

foldl op+ 0 a_list

in

if sum (

get[remote_host]( !remote_list_ref )

) > 40

then true

else false


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Example

• Remotely Finding List’s Sum (ML5)After:fun foo remote_host remote_list_ref =

let fun sum a_list =

foldl op+ 0 a_list

in

get[remote_host](

if sum ( !remote_list_ref ) > 40

then true

else false

)


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Types

• ML5 Type System Embeds a Notion of Place• Some values can be used at any

place.• e.g. Primitive data types, structures

• Some values can only be used at the location where they make sense.• e.g. File descriptors, reference cells,

printers


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Just a Few Types…

• τ@w – “The type τ is well-typed on host w.”


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Just a Few Types…

• get[w’,a]e – “Evaluate e on host w’ and return the result to the current host. Change e’s type from @w’ to @w.”

• Example:fun foo (x: int ref @w’, a: w’ addr @w) =

get[w’,a]( !x + !x )


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Just a Few Types…



get[w’,a]( !x + !x ) Typedint@w’


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get[w’,a]( !x + !x )

Just a Few Types…

Typedint@w


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Just a Few Types…

• □τ – “Suspended code that can be evaluated anywhere. Produces a value of type τ.”

• Example:(let fun sum il = foldl op+ 0 ilin

box (sum [1,2,3,4,5])end): □int @w


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Just a Few Types…

• ◊τ – “A value of type τ that exists at some other location.”

• Example:here (ref 5):◊(ref int) @w


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But What About Host Failure?

• What happens here?

(* at host 1 *)

get[w_2, a_2](

(* at host 2 *)

!int_ref_at_w_2 +

get[w_3, a_3](

(* at host 3 *)

!int_ref_at_w_3))


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(* at host 1 *)

get[w_2, a_2](

(* at host 2 *)

!int_ref_at_w_2 +

get[w_3, a_3](

(* at host 3 *)

!int_ref_at_w_3)) Host 2 dies!


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(* at host 1 *)

get[w_2, a_2](

(* at host 2 *)

!int_ref_at_w_2 +

get[w_3, a_3](

(* at host 3 *)


Throw an exception?


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(* at host 1 *)

get[w_2, a_2](

(* at host 2 *)

!int_ref_at_w_2 +

get[w_3, a_3](

(* at host 3 *)


Throw an exception?

Continue on from Host 3?


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(* at host 1 *)

get[w_2, a_2](

(* at host 2 *)

!int_ref_at_w_2 +

get[w_3, a_3](

(* at host 3 *)

!int_ref_at_w_3)

or_if_i_cant_return (...))) Host 2 dies!

Throw an exception?



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(* at host 1 *)

get[w_2, a_2](

(* at host 2 WHICH DOESN’T EXIST!*)

!int_ref_at_w_2 +

get[w_3, a_3](

(* at host 3 *)

!int_ref_at_w_3)

or_if_i_cant_return (...))) Host 2 dies!

Throw an exception?



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What We Want (Intuitively)

callcc x =>(* at host 1 *)get[w_2, a_2](

(* at host 2 *)

!int_ref_at_h_2 +get[w_3, a_3](

(* at host 3 *)!int_ref_at_h_3or_if_i_cant_return (throw (raise NetFail) to

x)))


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(* at host 2 *)



x)))

Don’t actually throw

something through the

network.


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(* at host 2 *)



x)))

Don’t actually throw

something through the

network.

Have host one detect the failure.


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Isn’t This Just a ‘Timeout’ Exception?

• A Good Question:• “Why not just have the ‘get’ operation

throw a timeout exception, like in Java?”

• e.g.

get[w_2, a_2] (

!int_on_w2

) handle TimeOut => (* do something *)


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Answers

1. This is actually a little smarter than just ‘timeout.’

2. The ‘Implicit Spawn’ Problem


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Answers



get[w_2, a_2] (

(* extremely complicated op *)



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Answers



get[w_2, a_2] (

(* extremely complicated op *)


T2

T1


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What We Need

• Share the Fact that Host 1 Has ‘Given Up’

• Kill the Thread ASAP• Make That Thread’s Actions

Irrelevant• Each host gets a chance to ‘undo’

potential effects.

• All with ‘Best Effort’


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One More Wrinkle

Catom 1

Catom 2

Grab ‘continuation’


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One More Wrinkle

Catom 1

Catom 2

Assign ‘Catom1’ to ‘myLeader’


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One More Wrinkle

Catom 1

Catom 2


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The Design, In Short

try

e_1

continuing

e_2

end


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try

e_1

continuing

e_2

end

1. Execute e_1


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try

e_1

continuing

e_2

end

1. Execute e_1

2. In the event of node failure... the entire expression will throw an exception on this host.


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try

e_1

continuing

e_2

end

1. Execute e_1

2. In the event of node failure... the entire expression will throw an exception on this host.

3. On the other hosts, e_2 will be executed, and its value discarded.


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(* host 1*)

try

(* set all of my neighbor’s

‘myLeader’ to host 1 *)

continuing

if !myLeader = host_1

then myLeader := NONE

else ()

end


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ML5-C: Error Continuations

Host

Visited Host

Location of thread

Migration of thread

PCtry

continuing

l:

end


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Host

Visited Host

Location of thread

Migration of thread

PC try

continuing

l:

end

Store Cont(stack)


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Host

Visited Host

Location of thread

Migration of thread

PCtry

continuing

l:

end

Store Cont(▪;l)


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Host

Visited Host

Location of thread

Migration of thread

PCtry

continuing

l:

end


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Host

Visited Host

Location of thread

Migration of thread

PC

try

continuing

l:

end

Store Cont(▪;l)


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Host

Visited Host

Location of thread

Migration of thread

PC

try

continuing

l:

end


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Host

Visited Host

Location of thread

Migration of thread

PC

try

continuing

l:

end


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Host

Visited Host

Location of thread

Migration of thread

PC

try

continuing

l:

end

Error!

Error!


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Host

Visited Host

Location of thread

Migration of thread

PC try

continuing

l:

end

Restore Cont.

Restore Cont.

PCl:


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Host

Visited Host

Location of thread

Migration of thread

PC raise Fail)

handle...

PC

l:


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Host

Visited Host

Location of thread

Migration of thread

PC

raise Fail)

handle...


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Interesting Note

• In Failure Case, We Have to Reason About Client and Server.• (The avoidance of this was one of the

touted benefits of ML5!)


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Future Work

• This Work is Not Yet Finished• More Restrictive Modal Basis

• Only neighbor catoms are accessible• This would be a ‘lower level’

language in some sense.

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Thanks!

Additional Questions?


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Failure Handling is More Natural

• In Claytronics, Failure is Possible at Any Moment.

• Intuitively, it would be nice to say:

try {

// a complex, multi host operation }

catch (Failure v) {

// take an alternate

// course of action. }

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So You Want to See the Typing Rules...

Note: These rules represent just a snapshot of the work.

Failure Handling in a modal Language

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