Edition 0 Updated to asp. Net core 0

public
static
IServiceCollection 
AddCustomHealthCheck
(
this
IServiceCollection services, 
IConfiguration configuration) 
{ 
var
accountName = configuration.
GetValue
<
string
>(
"AzureStorageAccountName"
); 
var
accountKey = configuration.
GetValue
<
string
>(
"AzureStorageAccountKey"
); 

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315 
CHAPTER 7 | Implement resilient applications 
var
hcBuilder = services.
AddHealthChecks
(); 
hcBuilder 
.
AddSqlServer
( 
configuration[
"ConnectionString"
], 
name: 
"CatalogDB-check"
, 
tags: 
new
string
[] { 
"catalogdb"
}); 
if
(!
string
.
IsNullOrEmpty
(accountName) && !
string
.
IsNullOrEmpty
(accountKey)) 
{ 
hcBuilder 
.
AddAzureBlobStorage
( 
$
"DefaultEndpointsProtocol=https;AccountName={accountName};AccountKey={accountKey};Endpoint
Suffix=core.windows.net"
, 
name: 
"catalog-storage-check"
, 
tags: 
new
string
[] { 
"catalogstorage"
}); 
} 
if
(configuration.
GetValue
<
bool
>(
"AzureServiceBusEnabled"
)) 
{ 
hcBuilder 
.
AddAzureServiceBusTopic
( 
configuration[
"EventBusConnection"
], 
topicName: 
"eshop_event_bus"
, 
name: 
"catalog-servicebus-check"
, 
tags: 
new
string
[] { 
"servicebus"
}); 
} 
else
{ 
hcBuilder 
.
AddRabbitMQ
( 
$
"amqp://{configuration["
EventBusConnection
"]}"
, 
name: 
"catalog-rabbitmqbus-check"
, 
tags: 
new
string
[] { 
"rabbitmqbus"
}); 
} 
return
services; 
} 
Finally, add the HealthCheck middleware to listen to “/hc” endpoint:
// HealthCheck middleware
app.
UseHealthChecks
(
"/hc"
, 
new
HealthCheckOptions
() 
{ 
Predicate = _ => 
true
, 
ResponseWriter = UIResponseWriter.
WriteHealthCheckUIResponse
}); 
Query your microservices to report about their health status 
When you’ve configured
health checks as described in this article and you have the microservice 
running in Docker, you can directly check from a browser if it’s healthy. You have to publish the 
container port in the Docker host, so you can access the container through the external Docker host IP 
or through 
host.docker.internal
, as shown in figure 8-8. 

---

316 
CHAPTER 7 | Implement resilient applications 
Figure 8-8. Checking health status of a single service from a browser 
In that test, you can see that the 
Catalog.API
microservice (running on port 5101) is healthy, 
returning HTTP status 200 and status information in JSON. The service also checked the health of its 
SQL Server database dependency and RabbitMQ, so the health check reported itself as healthy. 
Use watchdogs 
A watchdog is a separate service that can watch health and load across services, and report health 
about the microservices by querying with the 
HealthChecks
library introduced earlier. This can help 
prevent errors that would not be detected based on the view of a single service. Watchdogs also are a 
good place to host code that can perform remediation actions for known conditions without user 
interaction. 
The eShopOnContainers sample contains a web page that displays sample health check reports, as 
shown in Figure 8-9. This is the simplest watchdog you could have since it only shows the state of the 
microservices and web applications in eShopOnContainers. Usually a watchdog also takes actions 
when it detects unhealthy states. 
Fortunately, 
AspNetCore.Diagnostics.HealthChecks
 also provides 
AspNetCore.HealthChecks.UI
 NuGet 
package that can be used to display the health check results from the configured URIs. 

---

317 
CHAPTER 7 | Implement resilient applications 
Figure 8-9. Sample health check report in eShopOnContainers 
In summary, this watchdog service queries each microservice’s “/hc” end
point. This will execute all the 
health checks defined within it and return an overall health state depending on all those checks. The 
HealthChecksUI is easy to consume with a few configuration entries and two lines of code that needs 
to be added into the 
Startup.cs
of the watchdog service. 
Sample configuration file for health check UI: