Understanding our Isis Example Program

UNDERSTANDING OUR ISIS EXAMPLE PROGRAMKen Birman

Code for full programusing System;using System.Collections.Generic;using System.Linq;using System.Text;using Isis;using System.Threading;

namespace ConsoleApplication3{ public class tuple { public int rank; public int value;

public tuple(int r, int v) { rank = r; value = v; } }

class Program { static List<tuple> database = new List<tuple>(); public const int UPDATE = 0; public const int LOOKUP = 1; static void Main(string[] args) { IsisSystem.Start(); Group g = new Group("foo"); int myRank = 0; bool go = false, dbfull = false; ; g.ViewHandlers += (ViewHandler)delegate(View v) { Console.WriteLine("New View: " + v); myRank = v.GetMyRank(); if (v.members.Length == 3) go = true; }; g.Handlers[UPDATE] += (Action<int,int>)delegate(int rank, int n) { database.Add(new tuple(n, rank)); Console.WriteLine("New tuple: " + rank + "/" + n); if (database.Count() == 15) dbfull = true; };

g.Handlers[LOOKUP] += (Action<int>)delegate(int arg) { Console.WriteLine("=== Query for arg=" + arg); List<int> answer = new List<int>(); int index = 0; foreach (tuple tp in database) if (index++ % 3 == myRank) { Console.WriteLine("Looking at " + tp.rank + "/" + tp.value); if (tp.rank == arg) { Console.WriteLine("Including " + tp.rank + "/" + tp.value); answer.Add(tp.value); } } g.Reply(answer); }; g.Join(); while (!go) Thread.Sleep(10); for (int n = 0; n < 5; n++) g.OrderedSend(UPDATE, myRank, n); while (!dbfull) Thread.Sleep(10); if(myRank == 1) for (int n = 0; n < 3; n++) { List<List<int>> results = new List<List<int>>(); g.OrderedQuery(Group.ALL, LOOKUP, n, new Isis.EOLMarker(), results); Console.WriteLine("\r\nAnswers for Query rank=" + n); foreach (List<int> list in results) foreach (int value in list) Console.Write(value + " "); } IsisSystem.WaitForever(); } }}

What the program does Forms a process group When there are 3 members, each of the

3 simultaneously sends updates to the group

They store these updates in “tuple” objects, as a list, called the “database” list.

Then 2 members wait (Isis.WaitForEver) while the 3rd member sends queries to the list The queries are designed to select a subset

of the data We “subdivide” the work of searching the

list

What the program does First member starts

the system If several start at

same time, a Consensus within Isis2 decides which will be the first member

When it calls g.Join(),group is created

We see a first Viewreport

static void Main(string[] args) { IsisSystem.Start(); Group g = new Group("foo"); . . . g.Join(); while (!go) Thread.Sleep(1); . . . }

Consensus “in action”: IsisSystem.Start()

Step 1: Discovery “Is anyone running Isis?”

Step 2: Collision They hear each other! Consensus: The process

withthe smallest id is pickedby fault-tolerant algorithm

Step 3: A starts first. B and C are added next

Copy A B C Step 1

Step 2{A, B, C}

{A, B, C}

{A, B, C}Add

B Add C

B added

C added

Notice: A “wins the race” IsisSystem.Start()

finishes first in A

B and C join “in background” but user code is already running in A

Copy A B C Step 1

Step 2{A, B, C}

{A, B, C}

{A, B, C}Add

B Add C

B added

C added

A is active before B,C

FLP impact? It is impossible to guarantee progress for

a correct consensus protocol that tolerates even one fault

Clearly Isis2 startup is solving consensus

Thus we can guarantee that startup will succeed However, we can make it “very likely” that

it will succeed, and this is enough

What does A do now?

g.ViewHandlers += (ViewHandler)delegate(View v) { . . . Code for when a new membership view is defined . . . }; g.Handlers[UPDATE] += (Action<int,int>)delegate(int rank, int n) { . . . Code for doing an UPDATE action . . . }; g.Handlers[LOOKUP] += (Action<int>)delegate(int arg) { . . . Code for doing a LOOKUP action . . . }; g.Join();

Type signature

Anonymous method

“Delegate” concept In C++ every method must have a name

In Java and C# you don’t need to name a method if you are happy to use “anonymous” naming The compiler takes the “delegate” and

creates a method with a name the compiler itself assigns

Then it compiles this method

So we are simply registering a method with Isis2

What does A do now?


Variables in a delegate Arguments are provided during the

“upcall”

But a delegate can also access variables that were accessible in the method where it is placed

Thus our delegates can use variables created by the Main method. We use this to access the “database”

variable

Access to the database variable

class Program { static List<tuple> database = new List<tuple>(); public const int UPDATE = 0; public const int LOOKUP = 1; static void Main(string[] args) { . . . g.Handlers[UPDATE] += (Action<int,int>)delegate(int rank, int n) { database.Add(new tuple(n, rank)); Console.WriteLine("New tuple: " + rank + "/" + n); if (database.Count() == 15) dbfull = true; }; . . . } }

Declared “outside”

Access to the database variable

class Program { static List<tuple> database = new List<tuple>(); public const int UPDATE = 0; public const int LOOKUP = 1; static void Main(string[] args) { . . . g.Handlers[UPDATE] += (Action<int,int>)delegate(int rank, int n) { database.Add(new tuple(n, rank)); Console.WriteLine("New tuple: " + rank + "/" + n); if (database.Count() == 15) dbfull = true; }; . . . } }

Can be used “inside”

But the methods don’t run yet!


If group does not exist, creates it. A will run this first

What the program does So… the three

members try to start Isis2, but A “wins”

As a result, A runs the g.Join() first

A “new view” event occurs with 1 member

Isis: Searching for the Isis ORACLE...New View: View[gname=<foo>, gaddr=(0:224.0.69.120/0:0), viewid=0; 1 members={ [(5656)]}, hasFailed=[*-], nextIncomingMSGID={ <0:0> 0:0 }, StableTo={ ** 0 }, joining={ [(5656)]}, leaving={ []}, IamLeader = True

g.ViewHandlers += (ViewHandler)delegate(View v) { Console.WriteLine("New View: " + v); myRank = v.GetMyRank(); if (v.members.Length == 3) go = true; };

What is in the View object? Array of members (Address[n]), in v.members Who just joined (v.joined), left/failed (v.leaving) View id number, increments for each new view Group address: an Address object

Both groups and individual processes have Addresses

The “rank” of the member: 0..n-1 in the v.membersv.GetMyRank(), v.GetRankOf(someone)

What is in the View object? Group “foo” has Address

(0:224.0.69.120/0:0) There is one member: (5656). It just

joined. If “long format” is used, we would also see

the host IPv4 address: (5656:123.64.88.2:2345/1212)

The viewid is 0: the group was just created

Isis: Searching for the Isis ORACLE...New View: View[gname=<foo>, gaddr=(0:224.0.69.120/0:0), viewid=0; 1 members={ [(5656)]}, hasFailed=[*-], nextIncomingMSGID={ <0:0> 0:0 }, StableTo={ ** 0 }, joining={ [(5656)]}, leaving={ []}, IamLeader = True

Next B and C join the group New views are reported

Viewid 0: 1 member: A Viewid 1: 2 members: A, B Viewid 2: 3 members: A, B, C

Notice that ALL group members see the same view events, starting when they join

“Old” members have smaller rank than “new” ones

State transfer Our sample program didn’t transfer any state to the

joining member.

Homework task: Add state transfer State: contents of the List<Address> database In an Isis2 group, the state consists only of the state-

machine replicated data you associate with the group You do not need to include every variable!

In our sample program, the state was “empty” until after 3 members join, so state transfer was not needed

What did the program do next? Look closely at the View event handler

Each member Notes its “rank”, in the variable myRank

myRank=0 in A, 1 in B, 2 in C Sets boolean “go” to true when 3 members

are in the View that was just reported

g.ViewHandlers += (ViewHandler)delegate(View v) { Console.WriteLine("New View: " + v); myRank = v.GetMyRank(); if (v.members.Length == 3) go = true; };

What did the program do next? Now look at the Main method after g.Join

Every member loops, sleeping for 10ms at a time So it checks go 100 times/second When go becomes true, we pass the “barrier”

Homework: Replace “go” with a C# Semaphore: Semaphore go = new Semaphore(0, int.MaxValue); go.WaitOne() …. go.Release(1)

while (!go) Thread.Sleep(10);

What did the program do next? Now look at the Main method after

go==true

All three members concurrently begin to call g.OrderedSend!

for (int n = 0; n < 5; n++) g.OrderedSend(UPDATE, myRank, n);

A Sends… B Sends… C Sends…(0,0) (1,0) (2,0)(0,1) (1,1) (2,1)(0,2) (1,2) (2,2)(0,3) (1,3) (2,3)(0,4) (1,4) (2,4)

Isis2 has many multicast options g.SafeSend: Paxos, guarantees

Total ordering, all-or-nothing delivery, durability even if all members crash. Logged into a disk file

… this is stronger than we need! Our data is stored in memory (in the “database” List<tuple> variable)

g.OrderedSend: Optimistic early delivery Total ordering, all-or-nothing among

processes that do not crash. No logging, much faster than SafeSend

… this is what we are using

Isis2 has many multicast options g.Send: Optimistic, reliable, FIFO

First message sent is first message delivered But ordered only if sent by the same sender process:

A sends X, then A sends Y. X delivered before Y g.CausalSend: Optimistic, causal, reliable

Causal ordering First message sent is first delivered, even if different

senders (e.g. process B receives X from A, then sends Y. X delivered before Y because B sent Y after receiving X)

g.RawSend: No guarantees, not even reliable

With g.Send… Our multicasts could arrive in different orders

All database copies would have identical content but perhaps the order in the list would differ We saw this on Tuesday

With OrderedSend, all databases look the same State machine replication in “practice” Homework: In what ways does our application rely on ordering?

Would the code break if we used g.Send and not g.OrderedSend? Homework: Do we know what the ordering in database will be

before the program ran, or are there multiple possible outcomes?

Receiving the multicasts The members get upcalls to the multicast event

handler. In fact these are done on a different thread: the “multicast and view delivery thread”

dbfull becomes true when database has 15 items. Homework: replace dbfull with a Semaphore

g.Handlers[UPDATE] += (Action<int,int>)delegate(int rank, int n) { database.Add(new tuple(rank, n)); Console.WriteLine("New tuple: " + rank + "/" + n); if (database.Count() == 15) dbfull = true; };

What is a “tuple”? A simple object that stores a rank and a

value public class tuple { public int rank; public int value;


Could we put tuples in messages? Yes… but we would need to provide

“Marshalling” code, and a null constructor, like this:

Then must call Msg.RegisterType(typeof(tuple), 0); The 0 is a “unique type id” in range 0…128

[AutoMarshalled] public class tuple { public int rank; public int value;

public tuple(int r, int v) { rank = r; value = v; } public tuple() { } }

Can we put tuples in messages? Homework

Modify the test program to send tuples Store the received tuples into the database Thought question: Does UPDATE call “new

tuple”?

What happens next? Recall that tuple 15 sets dbfull to true Look at Main again

… all three programs (A, B and C) pass the while

g.OrderedSend(UPDATE, myRank, n); while (!dbfull) Thread.Sleep(10);

What happens next? Recall that tuple 15 sets dbfull to true Look at Main again

… all three programs (A, B and C) pass the while

g.OrderedSend(UPDATE, myRank, n); while (!dbfull) Thread.Sleep(10);

What happens next? Only member 1 (B) executes the last code:

B creates a “results” list, then issues an OrderedQuery asking for ALL responses It does this three times: with n=0, n=1, n=2 Each time it simply prints the list of responses

if(myRank == 1) for (int n = 0; n < 3; n++) { List<List<int>> results = new List<List<int>>(); g.OrderedQuery(Group.ALL, LOOKUP, n, new Isis.EOLMarker(), results); Console.WriteLine("\r\nAnswers for Query rank=" + n); foreach (List<int> list in results) foreach (int value in list) Console.Write(value + " "); }

What is a List<List<int>>?

In Java, C#, C++ we can create types that include other types are “arguments”. These are called Generics

The results variable is a List: it will have one value in it for each member of the group

… and that value will be of type List<int> Each group member sends a List in g.Reply

Finally… the query handler Look at the LOOKUP code:

g.Handlers[LOOKUP] += (Action<int>)delegate(int arg) { Console.WriteLine("=== Query for arg=" + arg); List<int> answer = new List<int>(); int index = 0; foreach (tuple tp in database) if (index++ % 3 == myRank) { Console.WriteLine("Looking at " + tp.rank + "/" + tp.value); if (tp.rank == arg) { Console.WriteLine("Including " + tp.rank + "/" + tp.value); answer.Add(tp.value); } } g.Reply(answer); };

Finally… the query handler … simplified version:


Each member… Has identical data in the database

(g.OrderedSend). … but has a different value of myRank:

0, 1 or 2Database item

A: myRank=0

B: myRank=1

C: myRank=2

0: (1,0) Looking at (1,0)

1: (0,0) Looking at (0,0)

2: (1,1) Looking at (1,1)

3: (2,0) Looking at (2,0)

4: (1,2) Looking at (1,2)

5: (0,1) Looking at (0,1)

Homework question Exactly what can we say about the order we

will see for tuples in database? Why are the copies identical in A, B and C? Did it matter that we only set go=true after A,B

and C all joined? Change the code to set go=true as soon as there

are two members, A and B Now use state transfer to fix the “bug” this causes

Will items sent by the same sender be in the sender order? (0,0)… (0,1)… (0,2)… (0,3)… (0,4)





A, B and C each scan 5 tuples Each one looks for rank==n, where n

was from the Query sender (n=0… then n=1… then n=2)

How many items will each one find? Thought question: how many items have

rank=0? … 5 have rank=0 But which member will “see” these tuples?

… we cannot know! Perhaps “luck” will cause all of these to

be scanned by A! But perhaps in a second run, A will see none

The situation depends on the data in database, and the order was not fully determined

Thus until we know the order, we cannot be sure which items A will look at, and until we know that, we cannot know how many will have rank=0

… but we DO know that Every tuple is scanned exactly once

And thus every rank=0 tuple will be seen by some member. Example: perhaps A finds (0,1) and (0,3) … and B finds none … while C finds (0,0), (0,2) and (0,4)

Each sends its own List<int>: A sends { 1, 3 }, etc

Back to the leader (process B) Again, look at code in Main

Results is a “List of List<int>”

if(myRank == 1) for (int n = 0; n < 3; n++) { List<List<int>> results = new List<List<int>>(); g.OrderedQuery(Group.ALL, LOOKUP, n, new Isis.EOLMarker(), results); Console.WriteLine("\r\nAnswers for Query rank=" + n); foreach (List<int> list in results) foreach (int value in list) Console.Write(value + " "); }

Back to the leader (process B) Again, look at code in Main

Perhaps results will have {1,3},{},{0,2,4}

So output will be 1 3 0 2 4

if(myRank == 1) for (int n = 0; n < 3; n++) { List<List<int>> results = new List<List<int>>(); g.OrderedQuery(Group.ALL, LOOKUP, n, new Isis.EOLMarker(), results); Console.WriteLine("\r\nAnswers for rank=" + n); foreach (List<int> list in results) foreach (int value in list) Console.Write(value + " "); }

What does “consistency” mean? Every tuple is scanned by one group member

We know that there is a 0, 1, 2, 3, 4 value for each rank value 0, 1, 2

So a consistent answer requires that we see 0..4 for each Query, but the program might not print in order

Homework: what we can say about the ordering within each List<int> reply element?

Which result came first? In the results list we do not know which part of the

result came from A, which from B and which from C

Homework: Add a second value to the reply giving the rank of the

member that sent each list Modify the output to show this informationAnswers for Query rank=0 [Member with myRank=0 sent {1,2}] [Member with myRank=2 sent {}] [Member with myRank=1 sent {0,3,4}]

Using the View Homework: Modify the program so that if the View

contains N members when the Query is done, each does 1/N of the work Currently code is “hard coded” to always use N=3

What if a failure causes loss of a Reply? Homework: Modify the replies to include value of N Modify the Main program to sense missing reply and

“retry” the Query: Missing a reply… must retry for rank=0 Homework: Test your fault-tolerance logic (hint: consider

using “IsisSystem.Terminate()” to “crash” a member)

Code for full programusing System;using System.Collections.Generic;using System.Linq;using System.Text;using Isis;using System.Threading;

namespace ConsoleApplication3{ public class tuple { public int rank; public int value;


class Program { static List<tuple> database = new List<tuple>(); public const int UPDATE = 0; public const int LOOKUP = 1; static void Main(string[] args) { IsisSystem.Start(); Group g = new Group("foo"); int myRank = 0; bool go = false, dbfull = false; ; g.ViewHandlers += (ViewHandler)delegate(View v) { Console.WriteLine("New View: " + v); myRank = v.GetMyRank(); if (v.members.Length == 3) go = true; }; g.Handlers[UPDATE] += (Action<int,int>)delegate(int rank, int n) { database.Add(new tuple(n, rank)); Console.WriteLine("New tuple: " + rank + "/" + n); if (database.Count() == 15) dbfull = true; };

g.Handlers[LOOKUP] += (Action<int>)delegate(int arg) { Console.WriteLine("=== Query for arg=" + arg); List<int> answer = new List<int>(); int index = 0; foreach (tuple tp in database) if (index++ % 3 == myRank) { Console.WriteLine("Looking at " + tp.rank + "/" + tp.value); if (tp.rank == arg) { Console.WriteLine("Including " + tp.rank + "/" + tp.value); answer.Add(tp.value); } } g.Reply(answer); }; g.Join(); while (!go) Thread.Sleep(10); for (int n = 0; n < 5; n++) g.OrderedSend(UPDATE, myRank, n); while (!dbfull) Thread.Sleep(10); if(myRank == 1) for (int n = 0; n < 3; n++) { List<List<int>> results = new List<List<int>>(); g.OrderedQuery(Group.ALL, LOOKUP, n, new Isis.EOLMarker(), results); Console.WriteLine("\r\nAnswers for Query rank=" + n); foreach (List<int> list in results) foreach (int value in list) Console.Write(value + " "); } IsisSystem.WaitForever(); } }}

Understanding our Isis Example Program

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