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Code patterns that can be used to increase reliability and resilience in your applications
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SystemReliability and
Resilienceand stuff
Some things need to be cleared up first
http://en.wikipedia.org/wiki/Vedette_(cabaret)
tuple
//Initialize customer and invoiceInitialize(customer, invoice);
public void Initialize(Customer customer, Invoice
invoice){
customer.Name = “asdf”;invoice.Date = DateTime.Now;
}
Initialize(customer, invoice);//did something happen to customer// and/or invoice?
customer.Name =
InitNameFrom(customer, invoice);invoice.Date =
InitDateFrom(customer, invoice);
customer.Name =
GetNameFrom(customer, invoice);invoice.Date =
GetDateFrom(customer, invoice);
var results = Initialize(customer,
invoice);
customer.Name = results.Item1;invoice.Date = results.Item2;
public tuple<string, DateTime>Initialize(customer,
invoice){
return new Tuple<string, DateTime>
(“asdf”, DateTime.Now);}
public static bool TryParse(string s, out DateTime result)
or
public static tuple<bool, DateTime?>
TryParse(string s)
tuple• Avoid side effects• Avoid out parameters•multiple values without a specific type
null object
private ILogger _logger;public MyClass(ILogger logger) {
_logger = logger;}
…
if (_logger != null) {_logger.Debug(
“it worked on my machine!”);}
null checks for everyone!
forget one and…
public class NullLogger : ILogger {
public void Debug(string text) {
//do sweet nothing}
}
private ILogger _logger = new NullLogger();
public MyClass(ILogger logger) {_logger = logger;
}
…
_logger.Debug(“it worked on my machine!”);
null object• Can eliminate null checks• Simple to implement
Circuit Breaker
Retry
You
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on O
ut o
f Pro
cess
Dep
en
dency
N times
Ou
t of P
roce
ss D
ep
en
dency
N times*
Y clients
= Denial of Service Attack
Limit the # of retries
N * Ybecomes
5 * Y
Y isstill a
problem
Circuit Breaker
State Machine
On :: Off
On Offwhen not healthy
Off Onmanually
Get to softwarebefore we ask you to dance
Healthyor
Unhealthy
Ou
t of P
roce
ss D
ep
en
dency
State is independent of requestor
Ou
t of P
roce
ss D
ep
en
dency
You
r A
pp
licati
on
Has many independent external dependencies
You
r A
pp
licati
on
Can throttle itself
You
r A
pp
licati
on
Has a wait threshold
Your Application
External Dependency
Circuit Breaker
Threshold = 2Pause = 10msTimeout = 30sState = ClosedRequest
Request
Failure (i.e. HTTP 500)Failure Count = 1Pause 10ms
Request
Failure (i.e. HTTP 500)Failure Count = 2State = Open
OperationFailedException
Threshold = 2Pause = 10msTimeout = 30sState = OpenRequest
30s has not passed
CircuitBreakerOpenException
Request
30s has not passed
CircuitBreakerOpenException
System can try to
become healthyfor 30s
Your Application
External Dependency
Circuit Breaker
Threshold = 2Pause = 10msTimeout = 30sState = ½ OpenRequest
Request
Failure (i.e. HTTP 500)
Failure Count = 2State = Open
OperationFailedException
30s has passed
Your Application
External Dependency
Circuit Breaker
Threshold = 2Pause = 10msTimeout = 30sState = ½ OpenRequest
Request
Failure Count = 0State = Closed
Response
30s has passed
Response
Your Application
External Dependency
Circuit Breaker
ClosedOpen
½ Open
½ Open is like a
manual reset
PauseTimeout
Pausebetween calls
in the loop
Timeoutbefore you
can call again
Exceptions
OperationFailed:
AggregateException
CircuitBreakerOpen:
ApplicationException
Don’t Loose Exception Info
Always use InnerException(s)
Threshold = 3State = ClosedRequest
Request
Failure (i.e. HTTP 500)Request
Failure (i.e. HTTP 500)Failure Count = 2
Failure Count = 0State = Closed
Response
Response
Request?
Your Application
External Dependency
Circuit Breaker
Failure Count = 1
SegregateDependencies
circuitBreaker(“database”)
circuitBreaker(“weatherservice”)
Dependency type, endpoint svc,
endpoint
Where?
You
r A
pp
licati
on O
ut o
f Pro
cess
Dep
en
dency
Cir
cuit
Bre
ake
r
Pro
xy
Watch forInception
You
r A
pp
licati
on W
eb
Serv
iceC
ircu
it B
reake
r
Cir
cuit
Bre
ake
r
Pro
xy
Data
base
Reposi
tory
circuit breaker• retry looping• slow down attempts• good neighbour
¡Muchas gracias!
