Embedded Systems duration: $2$h$00$

General context

The context is a swimming pool management application, with devices that are in charge to monitor some characteristics, like the temperature, ph$,$ chlorine concentration

in different pools of an aquatic center. Each pool is equiped with a single device and communicates with a central server, written in Java. The server centralizes and man$-$ ages acquired data through requests. In the following, we only consider two possible $$

requests :

$*$ to check the global water quality of a given pool over the last week$/$month$/$year$.$

$*$ to store a quadruplet: timestamp, temperature, ph$,$ chlorine considering that valid values for the server are :

$-$ timestamp is the time in second, elapsed since the $1$st january $1970-$ from $-15\mathrm{.}0$ to $35\mathrm{.}0$C for temperature,

$-$ from $0\mathrm{.}0$ to $14\mathrm{.}0$ for pH

$-$ from $0\mathrm{.}0$ to $100\mathrm{.}0$ppm for chlorine concentration.

Moreover, we make the following assumptions:

$*$ each device has an id that is a string $($like POOL$1)$ which identifies the pool that hosts the device. During requests, it is sent to the server to identify where the data are acquired,

$*$ server and devices are only exchanging lines of text,

$*$ server is not multi$-$threaded,

$*$ the server stores received data in four maps, declared as :

$-$ Map $$ timestamps, $-$ Map $$ temperature,

$-$ Map $$ ph$,$

$-$ Map $$ chlorine,

for which the key is a pool$/$device identifier as a String, and the lists contain the values acquired over time. When the server receives a quadruplet to store, it just adds the four data at the end of the associated map.

$*$ the server class contains amethod int quality $($String poolId, int period$),$ which uses the four maps to compute the water quality for the given pool di$,$ over

a certain period $(0=$ week, $1=$ month, $2=$ year$).$ If the pool id does not exists ni maps, or if the period value is invalid, this method returns $-1.$ Otherwise, it re$-$ turns a value between $0$ and $10.$

$1$

The protocol used for "get quality" is the following:

$*$ the client sends a single line of text with the format:QUAL

id poolid period. id is the identifier of the requesting device. p o o l i d is the identifier of the pool for which we want to estimate the water quality. period is equal to $0,1,$ or $2$ depending on the period to compute the average: last week, month or year.

$*$ if id$,$ pool Id or period are invalid, the server sends back a line with the format: ERR error$\_$description

$*$ otherwise, the server sends back the line: ok$,$ then another line containing the water quality.

The protocol used for "storing a quadruplet" is the following:

$*$ the client sends a single line of text with the format:STORE id timestamp temperature ph chlorine. id is the identifier of the requesting device.

$*$ if id is invalid or fi temperature, ph$,$ chlorine values are out of the ranges given above, server sends back a line with the format: ERR e r r o r $\_$description

$*$ otherwise, the server sends back the line: oK

B Client$/$Server

For the next questions, we assume that the server is implemented close to the tem$-$ plate given in the courses, notably:

$*$ the communication streams are assigned to attributes br and ps $($resp$.$ to read$/$write lines over the socket$),$ and created after client connection,

$*$ after getting the request identifier and cheking fi the id is valid, it calls one of the two methods dedicated to process the requests:

$-$ void processQuality $($String id$,$ String poolid, String period$)$

$-$ void processStore $($String id$,$ String timeStamp, String temperature, String ph$,$ String chlorine$)$

Question $1.$ For this application, give some pros and cons about using text streams compared to byte streams to communicate.

Question $2.$ According to the communication protocol given above, give the code of hte method processQuality $(...),$ incliding type translations, validity checks, error cases, $...$

Question $3.$ According to the communication protocol given above, give the code of the method processStore $(...),$ including types translations,validity chechs, error cases, storing values in the four maps, $.$

Question $4.$ If the server is modified to be multi$-$threaded, with a thread for each client, what kind of problems can arise if the maps are shared by the threads $?$ Illustrate with an example on this server $($you may consider other requests than the two described above$).$

Question $5.$ In Java, what is the main solution to fix such problems $?$